mt76: mt7615: fix potential overflow on large shift

Fix the following static checker warning:
error: undefined (user controlled) shift '(((1))) << (c->omac_idx)'

Fixes: 402a695b

 ("mt76: mt7615: fix CSA notification for DBDC")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>