pinctrl: stm32: return proper error code in pin_config_set

".pin_config_set" or ".pin_config_group_set" can be called with a
configuration not supported (i.e. PIN_CONFIG_PERSIST_STATE). In this case,
it is more suitable to return -ENOTSUPP instead of -EINVAL.

Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>
Link: https://lore.kernel.org/r/20200615125951.28008-2-alexandre.torgue@st.com


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