net: mvpp2: initialize the Tx FIFO size

So far only the Rx FIFO size was initialized. For PPv2.2 the Tx FIFO
size can be set as well. This patch initializes the Tx FIFO size for
PPv2.2 controllers to 3K.

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