w100fb: fix compile warnings

Fix compile warnings from w100fb.c.  Attribute registration failure doesn't
stop the driver working so just warn about it.

Signed-off-by: Richard Purdie <rpurdie@rpsys.net>
Signed-off-by: Antonino Daplas <adaplas@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>