staging: comedi: ni_labpc:use labpc_cancel() to remove some common code

The labpc_cancel() function can be used by the labpc_ai_insn_read() and
labpc_ai_cmd() functions to disable the ai subdevice.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>