net: nixge: Add support for 64-bit platforms

Add support for 64-bit platforms to driver.

The hardware only supports 32-bit register accesses
so the accesses need to be split up into two writes
when setting the current and tail descriptor values.

Signed-off-by: Moritz Fischer <mdf@kernel.org>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>