LoongArch: Fix unsigned comparison with less than zero

The return value from the call to get_timer_irq() is int, which can be
a negative error code. However, the return value is being assigned to an
unsigned int variable 'irq', so making 'irq' an int.

Eliminate the following coccicheck warning:
./arch/loongarch/kernel/time.c:146:5-8: WARNING: Unsigned expression compared with zero: irq < 0
Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

arch/loongarch/kernel/time.c

@@ -135,7 +135,7 @@ static int get_timer_irq(void)
 
 int constant_clockevent_init(void)
 {
-	unsigned int irq;
+	int irq;
 	unsigned int cpu = smp_processor_id();
 	unsigned long min_delta = 0x600;
 	unsigned long max_delta = (1UL << 48) - 1;