platform/x86: wmi: Make two functions static

Fix sparse warnings:

drivers/platform/x86/xiaomi-wmi.c:26:5: warning: symbol 'xiaomi_wmi_probe' was not declared. Should it be static?
drivers/platform/x86/xiaomi-wmi.c:51:6: warning: symbol 'xiaomi_wmi_notify' was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>