MIPS: convert IO port accessor functions to 'static inline'

MIPS: convert IO port accessor functions to 'static inline'

The currently used 'extern inline' directive causes
the following compiler warnings if CONFIG_SWAP_IO_SPACE
is defined:

  <...>/include/asm/io.h:345:1: warning: '__fswab32' is static but used in inline function '__outlc_p' which is not static [enabled by default]
  <...>/include/asm/io.h:345:1: warning: '__fswab32' is static but used in inline function '__outl_p' which is not static [enabled by default]
  <...>/include/asm/io.h:345:1: warning: '__fswab32' is static but used in inline function '__outlc' which is not static [enabled by default]
  <...>/include/asm/io.h:345:1: warning: '__fswab32' is static but used in inline function '__outl' which is not static [enabled by default]
  <...>/include/asm/io.h:344:1: warning: '__fswab16' is static but used in inline function '__outwc_p' which is not static [enabled by default]
  <...>/include/asm/io.h:344:1: warning: '__fswab16' is static but used in inline function '__outw_p' which is not static [enabled by default]
  <...>/include/asm/io.h:344:1: warning: '__fswab16' is static but used in inline function '__outwc' which is not static [enabled by default]
  <...>/include/asm/io.h:344:1: warning: '__fswab16' is static but used in inline function '__outw' which is not static [enabled by default]
  <...>/include/asm/io.h:341:1: warning: '__fswab32' is static but used in inline function '__inlc_p' which is not static [enabled by default]
  <...>/include/asm/io.h:341:1: warning: '__fswab32' is static but used in inline function '__inl_p' which is not static [enabled by default]
  <...>/include/asm/io.h:341:1: warning: '__fswab32' is static but used in inline function '__inlc' which is not static [enabled by default]
  <...>/include/asm/io.h:341:1: warning: '__fswab32' is static but used in inline function '__inl' which is not static [enabled by default]
  <...>/include/asm/io.h:340:1: warning: '__fswab16' is static but used in inline function '__inwc_p' which is not static [enabled by default]
  <...>/include/asm/io.h:340:1: warning: '__fswab16' is static but used in inline function '__inw_p' which is not static [enabled by default]
  <...>/include/asm/io.h:340:1: warning: '__fswab16' is static but used in inline function '__inwc' which is not static [enabled by default]
  <...>/include/asm/io.h:340:1: warning: '__fswab16' is static but used in inline function '__inw' which is not static [enabled by default]

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Cc: Daniel Schwierzeck <daniel.schwierzeck@googlemail.com>