phy: add phy_mac_interrupt() to use with PHY_IGNORE_INTERRUPT

There is currently no way for an Ethernet MAC driver servicing PHY link
interrupts to notify this to the PHY state machine without defining its
own state machine. Since most drivers are not so special, introduce a
helper: phy_mac_interrupt() which can be called from a link up/down
interrupt routine to update the PHY state machine. To avoid code
duplication some refactoring has been done to expose the workqueue and
its corresponding callback internally.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>