LIB82596: correct data types for hardware addresses

dma_addr_t is 64bit wide on some architectures (for example 64bit MIPS),
so it's not a good idea to use it for 32bit wide addresses in descriptors.

Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Jeff Garzik <jeff@garzik.org>