net: marvell: mvpp2: fix stuck in-band SGMII negotiation

It appears that the mvpp22 can get stuck with SGMII negotiation.  The
symptoms are that in-band negotiation never completes and the partner
(eg, PHY) never reports SGMII link up, or if it supports negotiation
bypass, goes into negotiation bypass mode (which will happen when the
PHY sees that the MAC is alive but gets no response.)

Triggering the PHY end of the link to re-negotiate results in the
bypass bit clearing on the PHY, and then re-setting - indicating that
the problem is at the mvpp22 GMAC end.

Asserting the GMAC reset and de-asserting it resolves the issue.
Arrange to assert the GMAC reset at probe time, and deassert it only
after we have configured the GMAC for the appropriate mode.  This
resolves the issue.

Tested-by: Sven Auhagen <sven.auhagen@voleatech.de>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>