ACPICA: Allow same ACPI table to be loaded/unloaded more than once

Without this change, a table cannot be loaded again once it has
been loaded/unloaded one time. The current mechanism does not
unregister a table upon an unload. During a load, if the same
table is found, this no longer returns an exception.

http://www.acpica.org/bugzilla/show_bug.cgi?id=722

Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Lin Ming <ming.m.lin@intel.com>
Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>