mt76: mt7615: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a
warning by replacing a /* fall through */ comment with the new
pseudo-keyword macro fallthrough; instead of letting the code fall
through to the next case.

Notice that Clang doesn't recognize /* fall through */ comments as
implicit fall-through markings.

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


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>