pinctrl: stm32: Print invalid AF warning inside stm32_pctrl_is_function_valid()

The "invalid function %d on pin %d .\n" message is triplicated in the
driver in different variants, just pull it into the function and have
it once in the driver. The bonus is that all variants of the message
now print the pin number and AF consistently, so it is easier to debug
such pinmux problems.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabien Dessenne <fabien.dessenne@st.com>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: linux-stm32@st-md-mailman.stormreply.com
To: linux-arm-kernel@lists.infradead.org
Acked-by: Fabien Dessenne <fabien.dessenne@foss.st.com>
Link: https://lore.kernel.org/r/20210406180035.279249-1-marex@denx.de


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>