net: nixge: Address compiler warnings when building for i386

Address compiler warning reported by kbuild autobuilders
when building for i386 as a result of dma_addr_t size on
different architectures.

warning: cast to pointer from integer of different size
	[-Wint-to-pointer-cast]

Fixes: 7e8d5755

 ("net: nixge: Add support for 64-bit platforms")
Signed-off-by: Moritz Fischer <mdf@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>