arch: riscv: smp: prevent nested #ifdef

The `CONFIG_PLIC_IRQ_AFFINITY` depends on `CONFIG_SMP` in the
Kconfig layer, nested #ifdef can be avoided for readability.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>
Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>

arch/riscv/core/smp.c

@@ -74,11 +74,11 @@ void arch_secondary_cpu_init(int hartid)
 #endif
 #ifdef CONFIG_SMP
 	irq_enable(RISCV_IRQ_MSOFT);
+#endif /* CONFIG_SMP */
 #ifdef CONFIG_PLIC_IRQ_AFFINITY
 	/* Enable on secondary cores so that they can respond to PLIC */
 	irq_enable(RISCV_IRQ_MEXT);
 #endif /* CONFIG_PLIC_IRQ_AFFINITY */
-#endif /* CONFIG_SMP */
 	riscv_cpu_init[cpu_num].fn(riscv_cpu_init[cpu_num].arg);
 }