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: NHulk Robot <hulkci@huawei.com>
Signed-off-by: NYueHaibing <yuehaibing@huawei.com>
Signed-off-by: NAndy Shevchenko <andriy.shevchenko@linux.intel.com>