net: phy: improve phy_driver_is_genphy

net: phy: improve phy_driver_is_genphy

Use new flag phydev->is_genphy_driven to simplify this function.
Note that this includes a minor functional change:
Now this function returns true if ANY of the genphy drivers
is bound to the PHY device.

We have only one user in DSA driver mt7530, and there the
functional change doesn't matter.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/c9ac3a7d-262a-425d-9153-97fe3ca6280a@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>