wifi: mt76: mt7615: do not advertise 5 GHz on first phy of MT7615D (DBDC)

On DBDC devices the first (internal) phy is only capable of using
2.4 GHz band, and the 5 GHz band is exposed via a separate phy object,
so avoid the false advertising.

Reported-by: Rani Hod <rani.hod@gmail.com>
Closes: https://github.com/openwrt/openwrt/pull/12361
Fixes: 7660a1bd

 ("mt76: mt7615: register ext_phy if DBDC is detected")
Cc: stable@vger.kernel.org
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Acked-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230605073408.8699-1-fercerpav@gmail.com