rt2x00: call sta_add/remove directly in rt2800

Only rt2800 subdriver of rt2x00 implement sta_add() and sta_remove(),
we do not need generic version of those.

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