staging: comedi: addi_apci_1564: fix s->maxdata assignment in do subdevice init.

s->maxdata for the do subdevice should be 1, however currently it is
being set to 0xffffffff.  Fix this.

Signed-off-by: Chase Southwood <chase.southwood@gmail.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>