mt76: connac: move mt76_connac_mcu_gen_dl_mode in mt76-connac module

Move mt76_connac_mcu_gen_dl_mode utility routine in mt76_connac_mcu.h
since it is shared by all drivers.

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