ALSA: usb-audio - Added functionality for E-mu 0404USB/0202USB/TrackerPre

Added functionality:
1) Extension Units support (all XU settings now available at alsamixer,
   kmix, etc):
- "AnalogueIn soft limiter" switch;
- "Sample rate" selector (values 0,1,2,3,4,5 corresponds to 44.1 48 ...
  192 kHz);
- "DigitalIn CLK source" selector (internal/external) (**);
- "DigitalOut format SPDIF/AC3" switch (**);
(**)E-mu-0404usb only.

2) Automatic device sample rate adjustment depending on substream
   samplerate for both capture and playback substream.

[minor coding-style fixes by tiwai]

Signed-off-by: Sergiy Kovalchuk <cnb_zerg@yahoo.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>