stifb: fix crash A1439A CRX (Rattler) graphics card

Fix kernel crash when stifb driver is used with a A1439A CRX (Rattler)
graphics card.  (Reference:
http://thread.gmane.org/gmane.linux.ports.hppa/1834)

Signed-off-by: Helge Deller <deller@gmx.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>