regmap: mdio: Reject invalid addresses

When an invalid register offset is provided, the upper bits are silently
discarded. Change this to return -ENXIO instead, to help catch potential
bugs.

Signed-off-by: Sander Vanheule <sander@svanheule.net>
Link: https://lore.kernel.org/r/047007e0e9fb596480829f11f8c7e6281d235c70.1623244066.git.sander@svanheule.net


Signed-off-by: Mark Brown <broonie@kernel.org>