staging: comedi: drivers.c: replace #include <linux/dma-mapping.h>

Comedi's "drivers.c" doesn't use anything from `<linux/dma-mapping.h>`,
but it does use `DMA_NONE` from `<linux/dma-direction.h>`, so replace
the appropriate `#include` directive.
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/drivers.c

@@ -23,7 +23,7 @@
 #include <linux/kernel.h>
 #include <linux/ioport.h>
 #include <linux/slab.h>
-#include <linux/dma-mapping.h>
+#include <linux/dma-direction.h>
 #include <linux/interrupt.h>
 #include <linux/firmware.h>