sony-laptop: fix a couple signedness bugs

This needs to be signed to handle negative error codes.
Remove a redundant check, read_limits is always called with a valid
handle.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mattia Dongili <malattia@linux.it>
Signed-off-by: Matthew Garrett <mjg@redhat.com>