net: mvpp2: set 802.3x GoP Flow Control mode

This patch fix GMAC TX flow control autoneg.
Flow control autoneg wrongly were disabled with enabled TX
flow control.

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