video/fbdev/stifb: Return -ENOMEM after a failed kzalloc() in stifb_init_fb()

Replace an error code for the indication of a memory allocation failure
in this function.

Fixes: 1da177e4 ("Linux-2.6.12-rc2: Initial git repository build")
Suggested-by: Rolf Eike Beer <eike-kernel@sf-tec.de>
Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Cc: Helge Deller <deller@gmx.de>
Cc: "James E. J. Bottomley" <jejb@parisc-linux.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>