net: phy: improve and inline phy_change

Now that phy_mac_interrupt() doesn't call phy_change() any longer it's
called from phy_interrupt() only. Therefore phy_interrupt_is_valid()
returns true always and the check can be removed.
In case of PHY_HALTED phy_interrupt() bails out immediately,
therefore the second check for PHY_HALTED including the call to
phy_disable_interrupts() can be removed.

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