mailbox: stm32-ipcc: check invalid irq

On failure of_irq_get() returns a negative value or zero, which is
not handled as an error in the existing implementation.
Instead of using this API, use platform_get_irq() that returns
exclusively a negative value on failure.
Also, do not output an error log in case of defer probe error.

Signed-off-by: Fabien Dessenne <fabien.dessenne@st.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>