dm: sound: make all functions static inline

dm: sound: make all functions static inline

Fixes following compile problem:
➜  u-boot-mainline git:(master) ✗ make sandbox_defconfig all NO_SDL=1
scripts/kconfig/conf  --syncconfig Kconfig
  CHK     include/config.h
  CFG     u-boot.cfg
  GEN     include/autoconf.mk
  GEN     include/autoconf.mk.dep
  CHK     include/config/uboot.release
  CHK     include/generated/version_autogenerated.h
  CHK     include/generated/timestamp_autogenerated.h
  UPD     include/generated/timestamp_autogenerated.h
  CHK     include/generated/generic-asm-offsets.h
  HOSTCC  tools/mkenvimage.o
  HOSTLD  tools/mkenvimage
  HOSTCC  tools/fit_image.o
  HOSTCC  tools/image-host.o
  HOSTCC  tools/dumpimage.o
  HOSTLD  tools/dumpimage
  HOSTCC  tools/mkimage.o
  HOSTLD  tools/mkimage
  HOSTLD  tools/fit_info
  HOSTLD  tools/fit_check_sign
  CC      cmd/version.o
  GZIP    cmd/config_data.gz
  CHK     cmd/config_data_gz.h
  CHK     cmd/config_data_size.h
  CHK     cmd/license_data_gz.h
  CHK     cmd/license_data_size.h
  LD      cmd/built-in.o
  CC      common/main.o
  LD      common/built-in.o
  CC      drivers/fastboot/fb_getvar.o
  LD      drivers/fastboot/built-in.o
  LD      drivers/video/built-in.o
ld.bfd: drivers/video/sandbox_sdl.o: in function `sandbox_sdl_sound_play':
/home/christian/projects/u-boot-mainline/./arch/sandbox/include/asm/sdl.h:110: multiple definition of `sandbox_sdl_sound_play'; drivers/video/video-uclass.o:/home/christian/projects/u-boot-mainline/./arch/sandbox/include/asm/sdl.h:110: first defined here
ld.bfd: drivers/video/sandbox_sdl.o: in function `sandbox_sdl_sound_init':
/home/christian/projects/u-boot-mainline/./arch/sandbox/include/asm/sdl.h:120: multiple definition of `sandbox_sdl_sound_init'; drivers/video/video-uclass.o:/home/christian/projects/u-boot-mainline/./arch/sandbox/include/asm/sdl.h:120: first defined here
make[3]: *** [scripts/Makefile.build:355: drivers/video/built-in.o] Fehler 1
make[2]: *** [scripts/Makefile.build:432: drivers/video] Fehler 2
make[1]: *** [Makefile:1531: drivers] Fehler 2
make: *** [Makefile:485: __build_one_by_one] Fehler 2

Fixes: f2b25c9bf82 ("dm: sound: Complete migration to driver model")
Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>