net: phy: add support for probing MMDs >= 8 for devices-in-package

Add support for probing MMDs above 7 for a valid devices-in-package
specifier, but only probe the vendor MMDs for this if they also report
that there the device is present in status register 2.  This avoids
issues where the MMD is implemented, but does not provide IEEE 802.3
compliant registers (such as the MV88X3310 PHY.)

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>