mt76: mt7615: report firmware version using ethtool

Print fw_ver and build_date members of struct mt7615_fw_trailer
similarly to what appears in the output of 'dmesg' when the MCU firmware
is loaded.

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