mt76: fix incorrect firmware path

mt76: fix incorrect firmware path

The mt76 driver does load the firmware for the MT7615 chip from
/lib/firmware/mediatek instead of /lib/firmware. The driver loads the
firmware from this path since mt76 commit
ea3ab68c7589 ("mt76: mt7615: fix mt7615 firmware path definitions").

Fixes: a2e2c40b5ebd ("mt76: update to the latest openwrt-19.07 version")
Reported-by: Stijn Segers <foss@volatilesystems.org>
Signed-off-by: David Bauer <mail@david-bauer.net>
Tested-by: Stijn Segers <foss@volatilesystems.org>