staging: comedi: drivers: move comedi_async 'cur_chan' tracking into the core

The commedi_async 'cur_chan' member is used to track the current position
in the chanlist for a scan. Currently only a couple comedi drivers use
this member.

For aeshtetics, move the 'cur_chan' tracking into the core for non-SDF_PACKED
subdevices. The 'cur_chan' will be updated after reading or writing samples
to the async buffer by comedi_inc_scan_progress(). All non-SDF_PACKED subdevices
will then automatiaclly track the 'cur_chan'.

Some of the drivers use the 'cur_chan' to detect the end of scan event when
counting scans. The COMEDI_CB_EOS event is automatically added by the core
when the end of scan is detected. The drivers just need to check if the
'cur_chan' is 0 to count the number of scans completed.

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>