staging: brcm80211: bugfix for NULL scb ptr dereference

The driver uses a struct called 'scb', this struct is primarily used for AMPDU
functionality and is embedded in struct ieee80211_sta. To increase driver
robustness, the case in which this scb pointer is NULL is now handled graceful.
This paves the way for the next patch in this series.

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>