mv643xx_eth: implement ethtool rx/tx ring size query and resizing

Rename the mp->default_[rt]x_ring_size variables to ->[rt]x_ring_size,
allow them to be read via the standard ethtool ->get_ringparam() op,
and add a ->set_ringparam() op to allow resizing them at run time.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>