staging: comedi: s626: use comedi_handle_events()

Use comedi_handle_events() to automatically (*cancel) the async command
for an end-of-acquisition or if an error/overflow occurs.

The s626_mc_disable() call when the end-of-acquisition is detected can
then be removed. The (*cancel) does the same thing.

For aesthetics, also remove the 'finished' local variable and just return
the ai_cmd_running state.

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>