bcm63xx: prevent fall-through in kernel patches for 5.4

bcm63xx: prevent fall-through in kernel patches for 5.4

In kernel 5.4 -Werror=implicit-fallthrough is treated as error:

arch/mips/bcm63xx/cpu.c: In function 'detect_cpu_clock':
arch/mips/bcm63xx/cpu.c:158:2: error: this statement may fall through [-Werror=implicit-fallthrough=]
  {
  ^
arch/mips/bcm63xx/cpu.c:179:2: note: here
  case BCM6328_CPU_ID:
  ^~~~
cc1: all warnings being treated as errors

This can be fixed by adding "Fall through" as a comment where intended,
and has to be fixed by returning a proper default otherwise.

In case of the default clock frequency for BCM6318 we fixed this by
returning the default value taken from BCM6328 and BCM6362.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>