pinctrl-sx150x: Improve sx150x_init_misc for SX1504/5/6

For Sx1504/5/6 only SX1506 has RegAdvanced, so put some code in place to
account for that.

Tested-by: Neil Armstrong <narmstrong@baylibre.com>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>