mt76: mt7663: add the possibility to load firmware v2

mt7663 firmware v2 is used for embedded devices since it has more completed
features in AP mode.
Add the capability to specify which firmware load first (v3 or v2)
using prefer_offload_fw kernel parameter and fallback to the other one
if the selected firmware fails to load

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>