mac80211: make mgmt_tx accept a NULL channel

cfg80211 passes a NULL channel to mgmt_tx if the frame has
to be sent on the one currently in use by the device.
Make the implementation of mgmt_tx correctly handle this
case. Fail if offchan is required.

Signed-off-by: Antonio Quartulli <antonio@open-mesh.com>
[fix RCU locking]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>