Blackfin arch: lookup channel2irq() only once

Add irq to struct dma_channel lookup channel2irq() only once,
since channel2irq() is fairly large on some Blackfin derivatives.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Bryan Wu <cooloney@kernel.org>

arch/blackfin/include/asm/dma.h

@@ -140,6 +140,7 @@ struct dma_channel {
 	struct dma_register *regs;
 	struct dmasg *sg;		/* large mode descriptor */
 	unsigned int ctrl_num;	/* controller number */
+	unsigned int irq;
 	dma_interrupt_t irq_callback;
 	void *data;
 	unsigned int dma_enable_flag;