net: netsec: use dma_addr_t for storing dma address

On targets that have different sizes for phys_addr_t and dma_addr_t,
we get a type mismatch error:

drivers/net/ethernet/socionext/netsec.c: In function 'netsec_alloc_dring':
drivers/net/ethernet/socionext/netsec.c:970:9: error: passing argument 3 of 'dma_zalloc_coherent' from incompatible pointer type [-Werror=incompatible-pointer-types]

The code is otherwise correct, as the address is never actually used as a
physical address but only passed into a DMA register.  For consistently,
I'm changing the variable name as well, to clarify that this is a DMA
address.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>