mt76x0: perform mt76x0_mac_set_ampdu_factor

Remove return added accidentally in mt76x0_mac_set_ampdu_factor.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: a7744349 ("mt76x0: mac files")
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>