DMA-API: dma: dw_dmac.c: convert to use dma_coerce_mask_and_coherent()

This code sequence:
	if (!pdev->dev.dma_mask) {
		pdev->dev.dma_mask = &pdev->dev.coherent_dma_mask;
		pdev->dev.coherent_dma_mask = DMA_BIT_MASK(32);
	}
bypasses the architectures check on the DMA mask.  It can be replaced
with dma_coerce_mask_and_coherent(), avoiding the direct initialization
of this mask.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>