irqchip/imx-intmux: Fix irqdata regs save in imx_intmux_runtime_suspend()

Gcc report warning as follows:

drivers/irqchip/irq-imx-intmux.c:316:29: warning:
 variable 'irqchip_data' set but not used [-Wunused-but-set-variable]
  316 |  struct intmux_irqchip_data irqchip_data;
      |                             ^~~~~~~~~~~~

irqdata regs is stored to this variable on the stack in
imx_intmux_runtime_suspend(), which means a nop. this commit
fix to save regs to the right place.

Fixes: bb403111 ("irqchip/imx-intmux: Implement intmux runtime power management")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200729155849.33919-1-weiyongjun1@huawei.com