staging: brcm80211: bugfix for control.sta NULL ptr dereference

Mac80211 can transmit packets where the control.sta field is NULL.
The driver dereferenced this. Bugfix was to only dereference a non NULL
ieee80211_sta pointer.

Signed-off-by: Roland Vossen <rvossen@broadcom.com>
Reviewed-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>