bus: firewall: Fix missing static inline annotations for stubs

bus: firewall: Fix missing static inline annotations for stubs

Stubs in the header file for !CONFIG_STM32_FIREWALL case should be both
static and inline, because they do not come with earlier declaration and
should be inlined in every unit including the header.

Cc: Patrice Chotard <patrice.chotard@foss.st.com>
Cc: stable@vger.kernel.org
Fixes: 5c9668cfc6d7 ("firewall: introduce stm32_firewall framework")
Link: https://lore.kernel.org/r/20250507092121.95121-2-krzysztof.kozlowski@linaro.org
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>