mt76x0: fix remove_interface

We wrongly use wcid_mask instead of vif_mask. This creates problems
when the interface is removed more than 8 times, for example with iw:

iw dev <devname> interface add <name> type <type>
iw dev <name> del

This caused 'ifconfig up' to fail with error:

SIOCSIFFLAGS: No space left on device

Fixes: 95e44409

 ("mt76x0: main file")
Reported-and-tested-by: Sid Hayn <sidhayn@gmail.com>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>