ALSA: usbmixer - add possibility to remap dB values

USB devices tends to represent dB ranges in different way than ALSA expects.
Add possibility to override these values and add guessed values for
SoundBlaster MP3+.

Also rename 'Capture Input Source' control to 'Capture Source' for
SoundBlaster MP3+ and Extigy.

Signed-off-by: Jaroslav Kysela <perex@perex.cz>