video: fbdev: stifb: handle NULL return value from ioremap_nocache

Add missing error check for ioremap_nocache() failure
(prevents NULL pointer dereference on error).

Cc: "James E.J. Bottomley" <jejb@parisc-linux.org>
Cc: Helge Deller <deller@gmx.de>
Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
[b.zolnierkie: minor fixes]
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>