net: phy: at803x: Use devm_regulator_get_enable_optional()

Use devm_regulator_get_enable_optional() instead of hand writing it. It
saves some line of code.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>