Add a new driver_caps bit, AZX_DCAPS_SNOOP_OFF, to set the snoop off
as default.  This new bit is used for the checks in
azx_check_snoop_available().  Most of case-switches are replaced with
the new dcaps in each entry.

While working on it, for avoiding to spend more bits, combine three
bits AZX_DCAPS_SNOOP_SCH, AZX_DCAPS_SNOOP_ATI and
AZX_DCAPS_SNOOP_NVIDIA bits into a flat type of two bits.  This
reduces the bits usages, and assign AZX_DCAPS_OFF to this empty bit
now.
Signed-off-by: Takashi Iwai <tiwai@suse.de>