[ALSA] ice1712 - disable unused ADCs & DACs on DMX6fire

Modules: ICE1712 driver

Change the value written to hardware configuration register of envy24 chip
in the case of Terratec DMX6fire to restrict the active ADCs & DACs to 6.

Also add the dxr_enable module option to eventually leave the old behaviour
when user requests.

Signed-off-by: Alan Horstmann <gineera@aspect135.co.uk>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>