video, udlfb: Fix two build warnings about 'ignoring return value'

build warning:

...
drivers/video/udlfb.c:1590: warning: ignoring return value of ‘device_create_file’, declared with attribute warn_unused_result
drivers/video/udlfb.c:1592: warning: ignoring return value of ‘device_create_bin_file’, declared with attribute warn_unused_result
...

So add two checks to get rid of 'em.

Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>