[MIPS] Use correct dma flushing in dma_cache_sync()

Not cache coherent R10k systems (like IP28) need to do real cache
invalidates in dma_cache_sync().

Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>