S3C2410: add error print if we cannot add attribute

Fix the following warning by checking the result of device_create_file and
printing an error but not removing the device (loss of debug registers is
not fatal).

drivers/video/s3c2410fb.c:905: warning: ignoring return value of 'device_create_file', declared with attribute warn_unused_result

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Cc: Arnaud Patard <arnaud.patard@rtp-net.org>
Acked-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>