While the AR9331 has a gigabit MAC towards the internal switch, the
integrated PHYs however are only 100-base-tx capable.  The existing code
however advertieses gigabit capability in the link status word.  If you
attach such a PHY to a gigabit capable switch on the remote end, with
some probability it attempts to negotiate gigabit and fails, falling
baco to the AR9331 assuming a 10mbit half-duplex link.  This has been
observed quite frequently with the Carambola2 and gigabit capable
switches.

In ath79_register_eth(), "pdata->has_gbit = 1;" is set unconditionally
for both AR9331 ethernet ports. This is most likely wrong. Despite the
two MAC IP cores being gigabit MACs, the MAC for eth1 is connected to a
100base-T PHY via MII. The has_gbit attribute is used in the ethernet
driver to determine the supported link modes.

So either pdata->has_gbit is not set to 1 anymore, or the ethernet
driver needs to be modified to determine the advertised link code word
on another criteria than pdata->has_gbit.  This patch implements the
former solution.

Signed-off-by: Harald Welte <laforge@gnumonks.org>

Backport of r42432

git-svn-id: svn://svn.openwrt.org/openwrt/branches/barrier_breaker@42434 3c298f89-4303-0410-b956-a3cf2f4a3e73