net: korina: Use DMA API

Instead of messing with MIPS specific macros use DMA API for mapping
descriptors and skbs.
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: David S. Miller <davem@davemloft.net>