ALSA: usb-audio: Integrate native DSD support for ITF-USB based DACs.

Integrate the native DSD support quirk codes of "ITF-USB DSD" based DACs.

Now, "is_itf_usb_dsd_2alts_dac()" and "is_itf_usb_dsd_3alts_dac()" is
integrated into one function "is_itf_usb_dsd_dac()".
So, remove the logic to distinguish UD-501 and UD-501V2 by the
"Product Name".

The integration is possible by changing the following two functions.

- snd_usb_select_mode_quirk():
Change the determination condition of the DSD mode switch command,
from the altset number being used, to the audio format being played.
Actually, this operation is same as playback using ASIO driver in
Windows environment.

- snd_usb_interface_dsd_format_quirk():
To which altset supports native DSD is determined by the number of altsets.
Previously, it's a constant "2" or "3".

Signed-off-by: Nobutaka Okabe <nob77413@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>