media: dvb_frontend: add proper __user annotations

Solves those warnings:
	drivers/media/dvb-core/dvb_frontend.c:2297:39: warning: incorrect type in argument 1 (different address spaces)
	drivers/media/dvb-core/dvb_frontend.c:2297:39:    expected void const [noderef] <asn:1>*<noident>
	drivers/media/dvb-core/dvb_frontend.c:2297:39:    got struct dtv_property *props
	drivers/media/dvb-core/dvb_frontend.c:2331:39: warning: incorrect type in argument 1 (different address spaces)
	drivers/media/dvb-core/dvb_frontend.c:2331:39:    expected void const [noderef] <asn:1>*<noident>
	drivers/media/dvb-core/dvb_frontend.c:2331:39:    got struct dtv_property *props

No functional changes.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>