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>

drivers/staging/comedi/drivers/ni_labpc_isadma.h

@@ -9,11 +9,23 @@
 
 #if NI_LABPC_HAVE_ISA_DMA
 
+static inline bool labpc_have_dma_chan(struct comedi_device *dev)
+{
+	struct labpc_private *devpriv = dev->private;
+
+	return (bool)devpriv->dma_chan;
+}
+
 int labpc_init_dma_chan(struct comedi_device *dev, unsigned int dma_chan);
 void labpc_free_dma_chan(struct comedi_device *dev);
 
 #else
 
+static inline bool labpc_have_dma_chan(struct comedi_device *dev)
+{
+	return false;
+}
+
 static inline int labpc_init_dma_chan(struct comedi_device *dev,
 				      unsigned int dma_chan)
 {