media: dvb/frontend.h: move out a private internal structure

struct dtv_cmds_h is just an ancillary struct used by the
dvb_frontend.c to internally store frontend commands.

It doesn't belong to the userspace header, nor it is used anywhere,
except inside the DVB core. So, remove it from the header.

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