staging: comedi: addi_apci_3120: remove private data 'ai_running'

This member of the private data was used to determine if the interrupt
routine was handling data for the (*insn_read) or an async command. Now
that the (*insn_read) does not use interrupts this member is not needed.

Remove the member and refactor 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>