zio_channel: added the current control for each channel

The current control used to be per-trigger (i.e., per-cset).
However, some parameters are per-channel (like the gain),
so each channel now has its own current control structure.

Also, zio-dump now shows device and trigger attributes, both standard
and
extended ones.

Signed-off-by: Federico Vaga <federico.vaga@gmail.com>
Acked-by: Alessandro Rubini <rubini@gnudd.com>