staging: comedi: ni_labpc: tidy up after DMA code migration

After migrating the ISA DMA handling code to the "ni_labpc_isadma"
module, get rid of an unneeded `#include` and a couple of unused
static variables.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>