ACPI: ipmi: remove useless return statement for void function

Remove useless return statement for void function, reported by
checkpatch.pl.

WARNING: void function return statements are not generally useful
FILE: drivers/acpi/acpi_ipmi.c:482:
+       return;
+}

Signed-off-by: Xiaofei Tan <tanxiaofei@huawei.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>