media: staging: atomisp: Remove unnecessary 'fallthrough'

commit df561f66 ("treewide: Use fallthrough pseudo-keyword") from
Gustavo A. R. Silva replaced and standardized /* fallthrough */ comments
with 'fallthrough' pseudo-keyword.

However, in one of the switch-case statements, Coverity Static Analyzer
throws a warning that 'fallthrough' is unreachable due to the adjacent
'return false' statement. (Coverity ID CID 1466511)

In order to fix the unreachable code warning, remove unnecessary
fallthrough keyword.

Signed-off-by: Cengiz Can <cengiz@kernel.wtf>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>