ACPICA: fix acpi_ex_release_mutex() comment

trivial, leftover from my NULL pointer dereference patch which got
'superseded' by commit fbc3be2a

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Len Brown <len.brown@intel.com>