brcm80211: make mgmt_tx in brcmfmac 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: brcm80211-dev-list@broadcom.com
Acked-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Antonio Quartulli <antonio@open-mesh.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>