HID: thrustmaster: fix sparse warnings

Changed 0 to NULL to fix following sparse warnings:

    drivers/hid/hid-thrustmaster.c:208:43: warning: Using plain integer as NULL pointer
    drivers/hid/hid-thrustmaster.c:241:17: warning: Using plain integer as NULL pointer
    drivers/hid/hid-thrustmaster.c:275:37: warning: Using plain integer as NULL pointer

Signed-off-by: Vihas Mak <makvihas@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>