ACPI / sysfs: Make function param_set_trace_method_name() static

The function param_set_trace_method_name is local to the source and does
not need to be in global scope, so make it static.

Cleans up sparse warning:
symbol 'param_set_trace_method_name' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>