Replace /* FALLTHROUGH */ comment with pseudo-keyword macro fallthrough[1]

[1] https://www.kernel.org/doc/html/latest/process/deprecated.html?highlight=fallthrough#implicit-switch-case-fall-throughAcked-by: Felipe Balbi <balbi@kernel.org>
Signed-off-by: Wei Ming Chen <jj251510319013@gmail.com>
Link: https://lore.kernel.org/r/20210505141936.4343-1-jj251510319013@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>