ramips: mt7621: replace missing custom-initramfs-uimage

custom-initramfs-uimage was replaced by calls to uImage, but apparently
mtc_wr1201 was missed in the transistion. Use uImage for this device
too.

Fixes: 9f574b1b "ramips: mt7621: drop custom uImage function"

Signed-off-by: Sander Vanheule <sander@svanheule.net>