sh: pci: Disable MWI and make pci_dma_burst_advice() a bit more accurate.

None of the SH PCI controllers support MWI, it is always treated as a
direct memory write, so simply disable it outright. In the case of the
PCI cache line size, consult that for the pci_dma_burst_advice()
strategy, and switch over to PCI_DMA_BURST_MULTIPLE, as PPC64.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>