net: phy: Invalidate LP advertising flags when restarting or disabling AN

It is possible to see the old value of the LP advertising flags
through ethtool after reconfiguring the PHY and before autonegotiation
completes.  If autonegotiation is turned off then the last value seen
will persist indefinitely.

Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>