acpi:ipmi: Convert ipmi_user_t to struct ipmi_user *

Signed-off-by: Corey Minyard <cminyard@mvista.com>
Cc: Zhao Yakui <yakui.zhao@intel.com>
Cc: Lv Zheng <lv.zheng@intel.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>