stm class: Fix module init return on allocation failure

In stm_heartbeat_init(): return value gets reset after the first
iteration by stm_source_register_device(), so allocation failures
after that will, after a clean up, return success. Fix that.

Fixes: 119291853038 ("stm class: Add heartbeat stm source device")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wang Hui <john.wanghui@huawei.com>
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Link: https://lore.kernel.org/r/20210115195917.3184-2-alexander.shishkin@linux.intel.com
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

stm class: heartbeat: Fix whitespace

Fix whitespace in the code for better readability, no functional changes.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Tested-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

stm class: Add SPDX GPL-2.0 header to replace GPLv2 boilerplate

This adds SPDX GPL-2.0 header to to stm core files and removes the
GPLv2 boilerplate text.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>

stm class: stm_heartbeat: Make nr_devs parameter read-only

Changing nr_devs after the module has been loaded doesn't actually
change anything, so just make it read-only.

Reported-by: Alan Cox <alan.cox@intel.com>
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Reviewed-by: Laurent Fert <laurent.fert@intel.com>

stm class: Add heartbeat stm source device

Heartbeat stm source may have multiple instances (for connecting to
different stm devices). Each instance will send a periodic test message
over its stm device when it is linked. This can be used for testing stm
class framework, stm device drivers or as a heartbeat over the stm link.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>