[media] pctv452e: fix sparse warnings

drivers/media/usb/dvb-usb/pctv452e.c:886:64: warning: Using plain integer as NULL pointer
drivers/media/usb/dvb-usb/pctv452e.c:903:63: warning: Using plain integer as NULL pointer
drivers/media/usb/dvb-usb/pctv452e.c:968:19: warning: Using plain integer as NULL pointer
drivers/media/usb/dvb-usb/pctv452e.c:1026:19: warning: Using plain integer as NULL pointer

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>