mt76: report firmware version using ethtool

print fw_ver and build_ver members of struct mt76x02_fw_header, similarly
to what appears in the output of 'dmesg' when the MCU firmware is loaded.

Reported-by: Sid Hayn <sidhayn@gmail.com>
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>