staging: comedi: comedidev.h: clarify async->event error/cancel detection

Introduce COMEDI_CB_ERROR_MASK and COMEDI_CB_CANCEL_MASK to clarify the
async->events that indicate errors and cancel an async command.

Use the new defines to tidy up and clarify the code.

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>