The comphy driver for Armada 3700 by Miquèl Raynal (which is currently
in linux-next) does not actually set comphy mode when phy_set_mode_ext
is called. The mode is set at next call of phy_power_on.

Update the driver to semantics similar to mvpp2: helper
mvneta_comphy_init sets comphy mode and powers it on.
When mode is to be changed in mvneta_mac_config, first power the comphy
off, then call mvneta_comphy_init (which sets the mode to new one).

Only do this when new mode is different from old mode.

This should also work for Armada 38x, since in that comphy driver
methods power_on and power_off are unimplemented.
Signed-off-by: Marek Behún <marek.behun@nic.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>