mt76: rename mt76x2_regs.h

MAC registers are the same for mt76x0 and mt76x2. Rename mt76x2_regs.h
file to reflect that.

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