dma: mv_xor: add missing free_irq() call

Even though the driver cannot be unloaded at the moment, it is still
good to properly free the IRQ handlers in the channel removal function.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
diff --git a/drivers/dma/mv_xor.h b/drivers/dma/mv_xor.h
index 698b448..1704328 100644
--- a/drivers/dma/mv_xor.h
+++ b/drivers/dma/mv_xor.h
@@ -81,6 +81,7 @@
 	spinlock_t		lock; /* protects the descriptor slot pool */
 	void __iomem		*mmr_base;
 	unsigned int		idx;
+	int                     irq;
 	enum dma_transaction_type	current_type;
 	struct list_head	chain;
 	struct list_head	completed_slots;