staging: comedi: ni_labpc_isadma: add labpc_have_dma_chan()

Add a static inline function to "ni_labpc_isadma.h" to test if a DMA
channel has been set-up, in which case `devpriv->dma_chan` will be
non-zero (where `devpriv` point to the private data for the comedi
device).  If the "ni_labpc_isadma" module is not being built, don't
bother checking `devpriv->dma_chan`; just return `false` as this may
help the compiler to optimize out some unreachable code.

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