remoteproc: sysmon: Update qcom_add_sysmon_subdev() comment

The comment says the qcom_add_sysmon_subdev() returns NULL on error but
it actually returns error pointers.

Signed-off-by: Dan Carpenter <[e-mail]>
Reviewed-by: Dmitry Baryshkov <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Bjorn Andersson <[e-mail]>

drivers/remoteproc/qcom_sysmon.c

···
  * @name:	name of this subdev, to use in SSR
  * @ssctl_instance: instance id of the ssctl QMI service
  *
- * Return: A new qcom_sysmon object, or NULL on failure
+ * Return: A new qcom_sysmon object, or an error pointer on failure
  */
 struct qcom_sysmon *qcom_add_sysmon_subdev(struct rproc *rproc,
 					   const char *name,