mt76: mt7615: introduce mt7615_mcu_send_ram_firmware routine

Add mt7615_mcu_send_ram_firmware routine since mt7615_load_ram runs the
same code to send ram firmware to cr4 and n9 mcus. Moreover rename
gen_dl_mode in mt7615_mcu_gen_dl_mode.
This patch does not introduce any behaviour change, it is just code
refactor.

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