[media] dvb: move dvb_set_frontend logic into a separate routine

This change is there in order to prepare the code to avoid calling
 dvb_frontend_ioctl_legacy() from FE_SET_PROPERTY.

A call to dvb_frontend_ioctl_legacy() would require to update the
DVBv3 cache without need, mangling calls for newer delivery system
without any reason.

No functional changes here.

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