riscv: Fix irq_work when SMP is disabled

irq_work is triggered via an IPI, but the IPI infrastructure is not
included in uniprocessor kernels. As a result, irq_work never runs.
Fall back to the tick-based irq_work implementation on uniprocessor
configurations.

Fixes: 29844792 ("riscv: Support irq_work via self IPIs")
Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20220430030025.58405-1-samuel@sholland.org
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/include/asm/irq_work.h

@@ -4,7 +4,7 @@
 
 static inline bool arch_irq_work_has_interrupt(void)
 {
-	return true;
+	return IS_ENABLED(CONFIG_SMP);
 }
 extern void arch_irq_work_raise(void);
 #endif /* _ASM_RISCV_IRQ_WORK_H */