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: a2e2c40b5e ("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>

package/kernel/mt76/Makefile

@@ -246,12 +246,12 @@ define KernelPackage/mt7603/install
 endef
 
 define KernelPackage/mt7615e/install
-	$(INSTALL_DIR) $(1)/lib/firmware
+	$(INSTALL_DIR) $(1)/lib/firmware/mediatek
 	cp \
 		$(PKG_BUILD_DIR)/firmware/mt7615_cr4.bin \
 		$(PKG_BUILD_DIR)/firmware/mt7615_n9.bin \
 		$(PKG_BUILD_DIR)/firmware/mt7615_rom_patch.bin \
-		$(1)/lib/firmware
+		$(1)/lib/firmware/mediatek
 endef
 
 $(eval $(call KernelPackage,mt76-core))