staging: comedi: pcl818: remove 'ai_mode' from private data

This member of the private data is not needed. We can determine the
interrupt mode based on the 'dma' and 'usefifo' members.

Refactor the interrupt handler and the (*cancel) function to not use
the 'ai_mode' and remove the setting of this member in the (*do_cmd)
heler functions.

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>