mac80211: fix warn, enum may be used uninitialized

regression introduced by b8d92c9c

In function ‘ieee80211_work_rx_queued_mgmt’:
warning: ‘rma’ may be used uninitialized in this function

this re-adds default value WORK_ACT_NONE back to rma

Signed-off-by: Christoph Fritz <chf.fritz@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>