wl12xx: remove redundant code from wl1271_op_conf_tx

Since the conf_tx callback passes the vif as param,
we must have been added first (and mac80211 verifies it).

Remove the handling of such case.

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>