mvneta: add forgotten initialization of autonegotiation bits

The commit 898b2970 ("mvneta: implement SGMII-based in-band link state
signaling")
changed mvneta_adjust_link() so that it does not clear the auto-negotiation
bits in MVNETA_GMAC_AUTONEG_CONFIG register. This was necessary for
auto-negotiation mode to work.
Unfortunately I haven't checked if these bits are ever initialized.
It appears they are not.
This patch adds the missing initialization of the auto-negotiation bits
in the MVNETA_GMAC_AUTONEG_CONFIG register.
It fixes the following regression:
https://www.mail-archive.com/netdev@vger.kernel.org/msg67928.html



Since the patch was tested to fix a regression, it should be applied to
stable tree.

Tested-by: Arnaud Ebalard <arno@natisbad.org>

CC: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
CC: Florian Fainelli <f.fainelli@gmail.com>
CC: netdev@vger.kernel.org
CC: linux-kernel@vger.kernel.org
CC: stable@vger.kernel.org

Signed-off-by: Stas Sergeev <stsp@users.sourceforge.net>
Signed-off-by: David S. Miller <davem@davemloft.net>