drivers/video/da8xx-fb.c: fix error return

The return should be negative on errors

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Cc: Sudhakar Rajashekhara <sudhakar.raj@ti.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>