MIPS: DMA: Fix computation of DMA flags from device's coherent_dma_mask.

This only matters for ISA devices with a 24-bit DMA limit or for devices
with a 32-bit DMA limit on systems with ZONE_DMA32 enabled.  The latter
currently only affects 32-bit PCI cards on Sibyte-based systems with more
than 1GB RAM installed.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>