net: phy: convert phy_stop() to use split state machine

Convert phy_stop() to use the new locked-section and unlocked-section
parts of the PHY state machine.

Tested-by: Jijie Shao <shaojijie@huawei.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>