PM / devfreq: Fix static checker warning in try_then_request_governor

The patch 23c7b54: "PM / devfreq: Fix devfreq_add_device() when
drivers are built as modules." leads to the following static checker
warning:

    drivers/devfreq/devfreq.c:1043 governor_store()
    warn: 'governor' can also be NULL

The reason is that the try_then_request_governor() function returns both
error pointers and NULL. It should just return error pointers, so fix
this by returning a ERR_PTR to the error intead of returning NULL.

Fixes: 23c7b54 ("PM / devfreq: Fix devfreq_add_device() when drivers are built as modules.")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>

drivers/devfreq/devfreq.c

@@ -228,7 +228,7 @@ static struct devfreq_governor *find_devfreq_governor(const char *name)
  * if is not found. This can happen when both drivers (the governor driver
  * and the driver that call devfreq_add_device) are built as modules.
  * devfreq_list_lock should be held by the caller. Returns the matched
- * governor's pointer.
+ * governor's pointer or an error pointer.
  */
 static struct devfreq_governor *try_then_request_governor(const char *name)
 {
@@ -254,7 +254,7 @@ static struct devfreq_governor *try_then_request_governor(const char *name)
  /* Restore previous state before return */
  mutex_lock(&devfreq_list_lock);
  if (err)
- return NULL;
+ return ERR_PTR(err);
 
  governor = find_devfreq_governor(name);
  }