linux-phytec-ti

mt76x0: load firmware from mediatek subdir

Firmware blob will be located in mediatek subdirectory. Add "u" suffix
to indicate this is USB device firmware.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>