pinctrl: sh-pfc: Use reg_width instead of reg as sentinel

All other loops over sh_pfc_soc_info.data_regs[] use
pinmux_data_reg.regwidth as the sentinel, which is safer as zero is
never a valid regwidth value (reg could be zero if we start using it to
store an offset).

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>