linux-firmware: move firmware file for mt7601u

linux-firmware: move firmware file for mt7601u

The firmware file for mt7601u (MediaTek MT7601U Wireless MACs) has
been moved to the mediatek/ folder by commit
 8451c2b1 mt76xx: Move the old Mediatek WiFi firmware to mediatek

Address this by updating the location of the firmware file in our
linux-firmware Makefile generating the mt7601u-firmware package.

All other MediaTek Wi-Fi firmware files are supplied by OpenWrt's
own repository rather than being taken from linux-firmware.

Fixes: d53fe5d9ce ("linux-firmware: update to 20230515")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
(cherry picked from commit d26ecbcf95e2c2ecf5380fdee733c6b46276e265)