staging: comedi: ni_labpc: migrate labpc_drain_dma()

Move `labpc_drain_dma()` into the "ni_labpc_isadma" module.  Provide a
dummy inline function in "ni_labpc_isadma.h" if the module is not being
built.

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