staging: comedi: ni_labpc: migrate DMA transfer set-up

Migrate the code for setting up an ISA DMA transfer into a new function
`labpc_setup_dma()` 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 `labpc_suggest_transfer_size()` 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>