staging: cxt1e1: Avoid using 0 instead of NULL

Fixed the following sparse warnings:

drivers/staging/cxt1e1/hwprobe.c:162:38:
	warning: Using plain integer as NULL pointer
drivers/staging/cxt1e1/hwprobe.c:163:38:
	warning: Using plain integer as NULL pointer
drivers/staging/cxt1e1/hwprobe.c:179:46:
	warning: Using plain integer as NULL pointer
drivers/staging/cxt1e1/hwprobe.c:184:46:
	warning: Using plain integer as NULL pointer

Signed-off-by: Thomas Vegas <thomas_75@safe-mail.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>