spapr: Fix integer overflow during migration (TCG)

The n_valid and n_invalid fields are unsigned short integers but it is
possible to have more than 65535 entries in a contiguous hunk, overflowing
the field. This results in an incorrect HTAB being sent to the destination
during migration.

Signed-off-by: Samuel Mendoza-Jonas <sam.mj@au1.ibm.com>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Alexander Graf <agraf@suse.de>