irqchip/loongson-htpic: Fix build warnings

Fix build warnings as below:

drivers/irqchip/irq-loongson-htpic.c: In function 'htpic_reg_init':
>> drivers/irqchip/irq-loongson-htpic.c:62:12: warning: variable 'val' set but not used [-Wunused-but-set-variable]
62 | uint32_t val;
| ^~~
drivers/irqchip/irq-loongson-htpic.c: At top level:
>> drivers/irqchip/irq-loongson-htpic.c:84:12: warning: no previous prototype for 'htpic_of_init' [-Wmissing-prototypes]
84 | int __init htpic_of_init(struct device_node *node, struct device_node *parent)
| ^~~~~~~~~~~~~

Fixes: a93f1d903fa34fc2c5d9fa450bd ("irqchip: Add driver for Loongson-3 HyperTransport PIC controller")
Reported-by: kernel test robot <[e-mail]>
Signed-off-by: Huacai Chen <[e-mail]>
Signed-off-by: Marc Zyngier <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

drivers/irqchip/irq-loongson-htpic.c

···
 	int i;
 
 	for (i = 0; i < HTINT_NUM_VECTORS; i++) {
-		uint32_t val;
-
 		/* Disable all HT Vectors */
 		writel(0x0, htpic->base + HTINT_EN_OFF + i * 0x4);
-		val = readl(htpic->base + i * 0x4);
+		/* Read back to force write */
+		(void) readl(htpic->base + i * 0x4);
 		/* Ack all possible pending IRQs */
 		writel(GENMASK(31, 0), htpic->base + i * 0x4);
 	}
···
 	.resume		= htpic_resume,
 };
 
-int __init htpic_of_init(struct device_node *node, struct device_node *parent)
+static int __init htpic_of_init(struct device_node *node, struct device_node *parent)
 {
 	unsigned int parent_irq[4];
 	int i, err;