staging: comedi: ni_labpc: migrate DMA channel init & free

Migrate the code for requesting an ISA DMA channel and a DMA buffer, and
the code for freeing them into two new functions in the
"ni_labpc_isadma" module: `labpc_init_dma_chan()` and
`labpc_free_dma_chan()`.  Dummy inline functions are provided in
"ni_labpc_isadma.h" if the "ni_labpc_isadma" module is not being built.

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