pinctrl: stm32: check for IRQ MUX validity during alloc()

Considering the following irq_domain_ops call chain:
- .alloc() is called when a clients calls platform_get_irq() or
  gpiod_to_irq()
- .activate() is called next, when the clients calls
  request_threaded_irq()
Check for the IRQ MUX conflict during the first stage (alloc instead of
activate). This avoids to provide the client with an IRQ that can't be
used.

Signed-off-by: Fabien Dessenne <fabien.dessenne@foss.st.com>
Link: https://lore.kernel.org/r/20210617144602.2557619-1-fabien.dessenne@foss.st.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>