acpi:ipmi: Make IPMI user handler const

ipmi_create_user() now takes the user handlers as const, make
it const in the ACPI IPMI code.

Cc: linux-acpi@vger.kernel.org
Cc: Zhao Yakui <yakui.zhao@intel.com>
Cc: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>
Acked-by: Rafael J. Wysocki <rjw@rjwysocki.net>