net: arc_emac: switch to phy_start()/phy_stop()

Currently driver use phy_start_aneg() in arc_emac_open() to bring
up PHY. But phy_start() function is more appropriate for this purposes.
Besides that it call phy_start_aneg() as part of PHY startup sequence
it also can correctly bring up PHY from error and suspended states.
So the patch replace phy_start_aneg() to phy_start().

Also the patch add call to phy_stop() to arc_emac_stop() to allow
the PHY device to be fully suspended when the interface is unused.

Signed-off-by: Alexander Kochetkov <al.kochet@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>