ray_cs: Use fallthrough pseudo-keyword

ray_cs: Use fallthrough pseudo-keyword

In order to enable -Wimplicit-fallthrough for Clang[1], replace the
existing /* fall through */ comments with the new pseudo-keyword
macro fallthrough[2].

[1] https://git.kernel.org/linus/84bd867e54befd4ce7b6b11c92c6a5884aebd69e
[2] https://www.kernel.org/doc/html/v5.7/process/deprecated.html?highlight=fallthrough#implicit-switch-case-fall-through

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20201008220422.GA6926@embeddedor