net: korina: Remove not needed cache flushes

Descriptors are mapped uncached so there is no need to do any cache
handling for them.

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>