ACPI / IPMI: Cleanup some initialization codes

This (trivial) patch.
 1. Changes dynamic mutex initialization to static initialization.
 2. Removes one acpi_ipmi_init() variable initialization as it is not
    needed.

Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Reviewed-by: Huang Ying <ying.huang@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>