V4L/DVB (6660): Allow fully configuring xc3028 during xc2028_attach

xc3028 can be used on some DTV only designs (for example, DVB-S boards). Before
this patch, a DTV only board would need to call set_tuner_config callback.

This patch allows to optionally pass a xc3028_ctrl parameter, via xc3028_config
struct, fully initializing the driver for DTV.

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>