mt76: mt7921: Fix fall-through warning for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix the
following warning by explicitly adding a break statement:

drivers/net/wireless/mediatek/mt76/mt7921/main.c:392:2: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]

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


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