intel_th: gth: Fix an off-by-one in output unassigning

commit 91d3f8a629849968dc91d6ce54f2d46abf4feb7f upstream.

Commit 9ed3f22223c3 ("intel_th: Don't reference unassigned outputs")
fixes a NULL dereference for all masters except the last one ("256+"),
which keeps the stale pointer after the output driver had been unassigned.

Fix the off-by-one.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Fixes: 9ed3f22223c3 ("intel_th: Don't reference unassigned outputs")
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>