nl80211: allow sending CMD_FRAME without specifying any frequency

Users may want to send a frame on the current channel
without specifying it.

This is particularly useful for the correct implementation
of the IBSS/RSN support in wpa_supplicant which requires to
receive and send AUTH frames.

Make mgmt_tx pass a NULL channel to the driver if none has
been specified by the user.

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