pinctrl: sx150x: Unregister the pinctrl on release

There is no matching call to pinctrl_unregister, so switch to the
managed devm_pinctrl_register to clean up properly when done.

Cc: stable@vger.kernel.org
Fixes: 9e80f906 ("pinctrl: Add SX150X GPIO Extender Pinctrl Driver")
Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>