pinctrl: stmfx: Improve error message in .remove()'s error path

pinctrl: stmfx: Improve error message in .remove()'s error path

The driver core has no handling for errors returned by the .remove()
callback. The only action on error is a dev_warn() with generic error
message that the returned value is returned.

Replace it by a more specific and useful message. Then returning zero is
the right thing to do, the only effect is to suppress the core's
warning.

This prepares the driver for the conversion to .remove_new().

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20231009083856.222030-2-u.kleine-koenig@pengutronix.de
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>