[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: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>