ALSA: usb-audio: Fix the mixer control range limiting issue

- mixer_ctl_set() function is limiting the volume level
  to particular range. This results in incorrect initial
  volume setting for that device.
- In USB mixer while calculating the dBmin/dBmax values
  resolution factor is hardcoded to 256 which results in
  populating the wrong values for dBmin/dBmax.
- Fix is to use appropriate resolution factor while
  calculating the dBmin/dBmax values.

CRs-Fixed: 515012
Change-Id: I502355af66f850bb65380c27333c3341fa43a947
Signed-off-by: Deepa Madiregama <dmadireg@codeaurora.org>