[media] radio-miropcm20: fix sparse NULL pointer warning

Fixes the following sparse warnings:

drivers/media/radio/radio-miropcm20.c:193:33: warning:
 Using plain integer as NULL pointer
Signed-off-by: NWei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: NHans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: NMauro Carvalho Chehab <m.chehab@samsung.com>

drivers/media/radio/radio-miropcm20.c

@@ -190,7 +190,7 @@ static int pcm20_setfreq(struct pcm20 *dev, unsigned long freq)
 	freql = freq & 0xff;
 	freqh = freq >> 8;
 
-	rds_cmd(aci, RDS_RESET, 0, 0);
+	rds_cmd(aci, RDS_RESET, NULL, 0);
 	return snd_aci_cmd(aci, ACI_WRITE_TUNE, freql, freqh);
 }