net: ethernet: cavium: octeon_mgmt: use phy_start and phy_stop

To start also "phy state machine", with UP state as it should be,
the phy_start() has to be used, in another case machine even is not
triggered. After this change negotiation is supposed to be triggered
by SM workqueue.

It's not correct usage, but it appears after the following patch,
so add it as a fix.

Fixes: 74a992b3 ("net: phy: add phy_check_link_status")
Signed-off-by: Ivan Khoronzhuk <ikhoronz@cisco.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>