irqchip/stm32: Fix non-SMP build warning

A CONFIG_SMP=n build emits a harmless compile-time warning:

drivers/irqchip/irq-stm32-exti.c:495:12: error: 'stm32_exti_h_set_affinity' defined but not used [-Werror=unused-function]

The #ifdef is inconsistent here, and it's better to use an IS_ENABLED() check
that lets the compiler silently drop that function.

Fixes: 927abfc4

 ("irqchip/stm32: Add stm32mp1 support with hierarchy domain")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ludovic Barre <ludovic.barre@st.com>
Cc: Rob Herring <robh@kernel.org>
Cc: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Cc: Radoslaw Pietrzyk <radoslaw.pietrzyk@gmail.com>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Link: https://lkml.kernel.org/r/20180605114347.1347128-1-arnd@arndb.de