staging: comedi: s626: cleanup "subdevice" and "cmd" in s626_irq_handler()

The comedi_subdevice in this function is actually the dev->read_subdev
that was initialized during the attach of the board. Use that instead
of accessing the dev->subdevices array directly.

Also, get the comedi_cmd used in s626_irq_handler() when the function
is first entered.

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