mt76: mt7615: fix mt7663e firmware struct endianness

Convert fields in mt7663_fw_trailer and mt7663_fw_buf to little-endian

Reported-by: kbuild test robot <lkp@intel.com>
Fixes: f40ac0f3 ("mt76: mt7615: introduce mt7663e support")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/d14dfd7cd91a4dda8c5dcd03e8a70ff11314182e.1584534454.git.lorenzo@kernel.org