mac80211: always clear SDATA_STATE_OFFCHANNEL flag

If the vif is stopped while it is offchannel (e.g. right
after p2p negotiation) the SDATA_STATE_OFFCHANNEL flag
is never get cleared, resulting in various bad effects
(e.g. GO can't start beaconing).

Fix it by clearing the SDATA_STATE_OFFCHANNEL flag
even if the vif is stopped.

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>