net: mvpp2: add ethtool flow control configuration support

This patch add ethtool flow control configuration support.

Tx flow control retrieved correctly by ethtool get function.
FW per port ethtool configuration capability added.

Patch also takes care about mtu change procedure, if PPv2 switch
BM pools during mtu change.

Signed-off-by: Stefan Chulski <stefanc@marvell.com>
Acked-by: Marcin Wojtas <mw@semihalf.com>
Signed-off-by: David S. Miller <davem@davemloft.net>