net: mvpp2: set the Rx FIFO size depending on the port speeds for PPv2.2

The Rx FIFO size was set to the same value for all ports. This patch
sets it depending on the maximum speed a given port can handle. This is
only working for PPv2.2.

Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>