mt76: mt7615: fix up typo in Kconfig for MT7663U

Fix up typo in Kconfig with indicating MT7663U is an 802.11ac device

Fixes: eb99cc95 ("mt76: mt7615: introduce mt7663u support")
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>