mt76: mt7615: add support for MT7611N

MT7611N is basically the same as MT7615N, except it only supports 5GHz
It is used by some TP-Link and Mercury wireless routers

Signed-off-by: DENG Qingfang <dqfext@gmail.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>