pinctrl: rzn1: Fix check for used MDIO bus

This fixes the check for unused mdio bus setting and the following static
checker warning:
 drivers/pinctrl/pinctrl-rzn1.c:198 rzn1_pinctrl_mdio_select()
 warn: always true condition '(ipctl->mdio_func[mdio] >= 0) => (0-u32max >= 0)'

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Phil Edworthy <phil.edworthy@renesas.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>