USB: dm9601: fix sparse NULL warnings

Fix sparse NULL warnings:

drivers/usb/net/dm9601.c:88:23: warning: Using plain integer as NULL pointer
drivers/usb/net/dm9601.c:174:22: warning: Using plain integer as NULL pointer

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>