r6040: invoke phy_{start,stop} when appropriate

Joe reported to me that right after a bring up of a r6040 interface
the ethtool output had no consistent output with respect to link duplex
and speed. Fix this by adding a missing phy_start call in r6040_up and
conversely a phy_stop call in r6040_down to properly initialize phy states.

Reported-by: Joe Chou <Joe.Chou@rdc.com.tw>
Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>