V4L/DVB: xc5000, rework xc_write_reg

Rework xc_write_reg function for correct read register of the xc5000.
It is very useful for tm6000.

Tested for tm6000 and for saa7134 works well.

Signed-off-by: Beholder Intl. Ltd. Dmitry Belimov <d.belimov@gmail.com>
Acked-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>