alsa: Scan all soundcards and create a sigrok device per input

alsa: Scan all soundcards and create a sigrok device per input

The alsa driver only works with device "default". This limits the driver's
scope to whatever device ALSA deems to be "default". It is desirable to have
access to all ALSA devices from sigrok.

Change the alsa device scan so that:
Each alsa device (not alsa card) gets its own sigrok device
For example,
    hw:1,0 == sigrok device 0
    hw:1,1 == sigrok device 1
    hw:2,0 == sigrok device 2
    hw:2,1 == sigrok device 3
    hw:2,2 == sigrok device 4
    [...]

We don't currently look at alsa subdevices. We only use subdevice 0.
Every input device will have its own channels (left, right, etc). Each of
those channels gets mapped to a different sigrok probe. A device with 4
channels will have 4 probes from sigrok's perspective.

Signed-off-by: Alexandru Gagniuc <redacted>