iwlwifi: mvm: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix multiple
warnings by explicitly using the fallthrough pseudo-keyword as a
replacement for a number of "fall through" markings.

Notice that Clang doesn't recognize "fall through" comments as
implicit fall-through.

Link: https://github.com/KSPP/linux/issues/115


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20201117135053.GA13248@embeddedor