mt76: create new mt76x02-lib module for common mt76x{0,2} code

Move rxfilter and mutex to common structure. Create mt76x02-lib.ko
for mt76x0 and mt76x2 common functions and create new unified
mt76x02_configure_filter() function there.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>