irqchip/stm32-exti: Use the hwspin_lock_timeout_in_atomic() API

Now that the hwspin_lock_timeout_in_atomic() API is available use it.

Signed-off-by: Fabien Dessenne <fabien.dessenne@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200706081115.25180-1-alexandre.torgue@st.com