ath6kl: 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

Cc: Nicolas Cavallari <Nicolas.Cavallari@lri.fr>
Acked-by: Kalle Valo <kvalo@qca.qualcomm.com>
Signed-off-by: Antonio Quartulli <antonio@open-mesh.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>