net: phy: improve handling delayed work

Using mod_delayed_work() allows to simplify handling delayed work and
removes the need for the sync parameter in phy_trigger_machine().
Also introduce a helper phy_queue_state_machine() to encapsulate the
low-level delayed work calls. No functional change intended.

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