net: phy: Add is_on_sfp_module flag and phy_on_sfp helper

Add a flag and helper function to indicate that a PHY device is part of
an SFP module, which is set on attach. This can be used by PHY drivers
to handle SFP-specific quirks or behavior.

Signed-off-by: Robert Hancock <robert.hancock@calian.com>
Signed-off-by: David S. Miller <davem@davemloft.net>