video: fbdev: au1200fb: Return an error code if a memory allocation fails

'ret' is known to be 0 at this point.
In case of memory allocation error in 'framebuffer_alloc()', return
-ENOMEM instead.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>