acpi_ec: clean up 'private' ivar when freeing memory to which it points
acpi_ec: clean up 'private' ivar when freeing memory to which it points

This is not only a prudent thing to do, but also makes sure that probe
method is not confused by non-NULL 'private', if the previous attach
attempt fails for any reason.

PR:		kern/142561
Tested by:	Alex Goncharov <alex-goncharov@comcast.net>
MFC after:	4 days

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase