Staging: slicoss: changes comparisons to NULL in slicoss.c

This is a patch to slicoss.c to change the style of NULL comparisons in
order to remove checkpatch.pl warnings.

Signed-off-by: Ben Marsh <bmarsh94@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>