When reboot system, no power cycles, firmware is already downloaded,
return -EIO will break driver as error:
mt7921e: probe of 0000:03:00.0 failed with error -5

Skip firmware download and continue to probe.
Signed-off-by: Aaron Ma <aaron.ma@canonical.com>
Fixes: 1c099ab4 ("mt76: mt7921: add MCU support")
Signed-off-by: David S. Miller <davem@davemloft.net>