staging: comedi: ni_labpc: migrate DMA status handling

Migrate the code for checking and handling the interrupt status handling
for ISA DMA transfers into new a new function
`labpc_handle_dma_status()` in the "ni_labpc_isadma" module.  Provide a
dummy inline function in "ni_labpc_isadma.h" if the "ni_labpc_isadma"
module is not being built.

The static function `handle_isa_dma()` also needs to move across to the
new module.

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