[media] drxd_hard: fix sparse warnings

drivers/media/dvb-frontends/drxd_hard.c:1017:70: warning: Using plain integer as NULL pointer
drivers/media/dvb-frontends/drxd_hard.c:1038:69: warning: Using plain integer as NULL pointer
drivers/media/dvb-frontends/drxd_hard.c:2836:33: warning: Using plain integer as NULL pointer
drivers/media/dvb-frontends/drxd_hard.c:2972:30: warning: Using plain integer as NULL pointer

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Antti Palosaari <crope@iki.fi>
Reviewed-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>