mt76: mt7921: remove unneeded semicolon

Fix the following coccicheck warnings:

./drivers/net/wireless/mediatek/mt76/mt7921/mac.c:1402:2-3: Unneeded
semicolon.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>