video: fbdev: pxafb: handle errors from pxafb_init_fbinfo() correctly

pxafb_init_fbinfo() can not only report errors caused by failed
allocations but also when the clock can't be found.

To fix this, return an error pointer instead of NULL in case of errors,
and up-chain the result.

Signed-off-by: Daniel Mack <daniel@zonque.org>
Reviewed-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>