mt76: mt7915: rely on mt76_connac_mcu_add_tlv routine

Rely on common code available in mt76-connac module and remove
mt7915_mcu_add_tlv/mt7915_mcu_add_nested_tlv in mt7915 driver.

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