net: phy: constify phydev->drv

net: phy: constify phydev->drv

Device driver structures are shared between all devices that they
match, and thus nothing should never write to the device driver
structure through the phydev->drv pointer. Let's make this pointer
const to catch code that attempts to do so.

Suggested-by: Christian Marangi <ansuelsmth@gmail.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://lore.kernel.org/r/E1rVxXt-002YqY-9G@rmk-PC.armlinux.org.uk
Signed-off-by: Paolo Abeni <pabeni@redhat.com>