net: phy: at803x: disable vddio regulator

The probe() might enable a VDDIO regulator, which needs to be disabled
again before calling regulator_put(). Add a remove() function.

Fixes: 2f664823 ("net: phy: at803x: add device tree binding")
Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>