net: mvpp2: fix 10 Mbit/s usage

This commit is similar to commit 4d12bc63 ("net: mvneta: fix
operation in 10 Mbit/s mode"), but this time for the mvpp2 driver. The
driver was properly taking into account the 1 Gbit/s and 100 Mbit/s
speeds, but not the 10 Mbit/s, which was handled as 100
Mbit/s. However, the MVPP2_GMAC_CONFIG_MII_SPEED bit in the
MVPP2_GMAC_AUTONEG_CONFIG register must remain cleared to allow 10
Mbit/s operation. This commit therefore fixes 10 Mbit/s operation.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>