staging: comedi: pcmuio: remove 'asic_chan'

This member of the subdevice private data is always initialize to
'0' due to the 'thisasic_chanct' always being zero when it is set
during the attach. Simplify the driver a bit by removing it.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>