net: sh_eth: do not advertise Gigabit capabilities when not available

Not all variants of the sh_eth hardware have Gigabit
support. Unfortunately, the current driver doesn't tell the PHY about
the limited MAC capabilities. Due to this, if you have a Gigabit
capable PHY, the PHY will advertise its Gigabit capability and
establish a link at 1Gbit/s, even though the MAC doesn't support it.

In order to avoid this, we use the recently introduced
phy_set_max_speed() to tell the PHY to not advertise speed higher than
100 MBit/s.

Tested on a SH7786 platform, with a Gigabit PHY.

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