sgiseeq: replace use of dma_cache_wback_inv

The sgiseeq driver is one of the few remaining users of the ancient
cache banging DMA API.  Replaced with the modern days DMA API.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>