net: phy: warn if phy_start is called from invalid state

phy_start() should be called from states PHY_READY or PHY_HALTED only.
Check for this to detect misbehaving drivers. Also the state machine
should be started only when being called from one of the valid states.

Some more background:
For all invalid states phy_start() basically was a no-op. All it did
was triggering a state machine run, but for all "running" states the
poll loop was active anyway. And if called from PHY_DOWN, the state
machine does nothing.

v3:
- extended commit message

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>