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: a93f1d90 ("irqchip: Add driver for Loongson-3 HyperTransport PIC controller")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Huacai Chen <chenhuacai@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/1607159744-995-1-git-send-email-chenhuacai@kernel.org