powerpc/viobus: Free TCE table on device release

Release the TCE table as the XXX suggests, except on FW_FEATURE_ISERIES,
where the tables are allocated globally and reused.

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Nishanth Aravamudan <nacc@us.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>