coresight: stm: return error code instead of zero in .packet()

coresight: stm: return error code instead of zero in .packet()

In STM framework driver, the trace data writing loop would keep running
until it received a negative return value or the whole trace packet has
been written to STM device.  So if the .packet() of STM device always
returns zero since the device is not enabled or the parameter isn't
supported, STM framework driver will stall into a dead loop.

Returning -EACCES (Permission denied) in .packet() if the device is
disabled makes more sense, and this is the same for returning -EINVAL
if the channel passed into is not supported.

Signed-off-by: Chunyan Zhang <zhang.chunyan@linaro.org>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>