[ARM] Export dma_channel_active()

dma_channel_active() is used by some modules and is part of our
DMA API, so export it.
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>

arch/arm/kernel/dma.c

@@ -228,6 +228,7 @@ int dma_channel_active(dmach_t channel)
 {
 	return dma_chan[channel].active;
 }
+EXPORT_SYMBOL(dma_channel_active);
 
 void set_dma_page(dmach_t channel, char pagenr)
 {