mt76: mt7615: defer mcu initialization via workqueue

Loading the mcu firmware and waiting for it to boot takes a long time,
which adds a significant amount to the system boot time.
Fix this by running the mcu init from a workqueue and waiting for it to
complete before starting the phy or issuing mcu commands via debugfs

Signed-off-by: Felix Fietkau <nbd@nbd.name>