video: fbdev: au1200fb: Propagate an error code

We should propagate the error code returned by 'fb_alloc_cmap()' instead
of returning -EFAULT.

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