platform/x86: thinkpad_acpi: Make some symbols static

The sparse tool complains as follows:

drivers/platform/x86/thinkpad_acpi.c:9636:25: warning:
 symbol 'dev_attr_charge_start_threshold' was not declared. Should it be static?
drivers/platform/x86/thinkpad_acpi.c:9642:25: warning:
 symbol 'dev_attr_charge_stop_threshold' was not declared. Should it be static?

Those variables are not used outside of thinkpad_acpi.c, so this
commit marks them static.

Fixes: e3392953 ("platform/x86: thinkpad_acpi: use standard charge control attribute names")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>