mt76: mt7921: move mt7921_init_hw in a dedicated work

Firmware initialization can take a while. Move mt7921_init_hw routine in
a dedicated work in order to not slow down bootstrap process.

Tested-by: Deren Wu <deren.wu@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>