acpi_tb_delete_namespace_by_owner() expects ACPI_MTX_INTERPRETER to be
taken. This fixes the following issue:

ACPI Error: Mutex [0x0] is not acquired, cannot release (20141107/utmutex-322)
Call Trace:
  [<ffffffff81b0bd28>] dump_stack+0x4f/0x7b
  [<ffffffff81546bfc>] acpi_ut_release_mutex+0x47/0x67
  [<ffffffff81542cf1>] acpi_tb_delete_namespace_by_owner+0x57/0x8d
  [<ffffffff81543ef1>] acpi_unload_table_id+0x3a/0x5e
Signed-off-by: NOctavian Purdila <octavian.purdila@intel.com>
Signed-off-by: NRafael J. Wysocki <rafael.j.wysocki@intel.com>