staging: unisys: fix comparison to NULL in visorchipset.c

This patch fixes the following type of check patch warnings:
Comparison to NULL could be written

Signed-off-by: Erik Arfvidson <erik.arfvidson@unisys.com>
Signed-off-by: Benjamin Romer <benjamin.romer@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>