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>