mt76: fix build for MediaTek MT7610U USB wireless dongle

The mt76x0 driver requires the mt76 core driver to actually function.
So add a 'select' to avoid embarrassing 'symbol unknown' errors
when attempting to modprobe it in a module tree that doesn't
include mt76.ko

Signed-off-by: Valdis Kletnieks <valdis.kletnieks@vt.edu>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>