sh: Quiet noisy coherent DMA unmapping on R2D.

IRQs are re-enabled at a later stage when doing the unmapping on R2D via
the sm501 USB coherent DMA, resulting in the irqs_disabled() check
producing considerable noise for this configuration. Just kill off the
check, which was blindly copied from x86 anyways.
Signed-off-by: NPaul Mundt <lethal@linux-sh.org>

arch/sh/include/asm/dma-mapping.h

@@ -89,8 +89,6 @@ static inline void dma_free_coherent(struct device *dev, size_t size,
 {
 	struct dma_map_ops *ops = get_dma_ops(dev);
 
-	WARN_ON(irqs_disabled());	/* for portability */
-
 	if (dma_release_from_coherent(dev, get_order(size), vaddr))
 		return;