coresight: stm: Free channels in stm_output_disclaim

output->nr_chans was set to zero before adding to
master->nr_free. master->nr_free is not set correctly.
This change adds output->nr_chans to master->nr_free
first and sets output->nr_chans to zero.

Change-Id: I901125c1f18e091652b78c5db39ba206af23fdd2
Signed-off-by: Tingwei Zhang <tingwei@codeaurora.org>