iwlwifi: fix userspace setting of sleep_level_override

The sleep_level_override debugfs file is used by the user to request a
static power index instead of the dynamic sleep values. Users are expected
to provide value from 1 to 5 as an index or -1 to disable it.

The problem at the moment is that users can also provide 0 to this file
which, together with the value 1, is translated to index 1. This is
confusing and even more so when users write 0 to sleep_level_override and
then read 1 from it afterwards.

Modify checking to treat 0 as invalid.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/iwlwifi/iwl-debugfs.c

@@ -801,7 +801,9 @@ static ssize_t iwl_dbgfs_sleep_level_override_write(struct file *file,
 	 * valid here. However, let's not confuse them and present
 	 * IWL_POWER_INDEX_1 as "1", not "0".
 	 */
-	if (value > 0)
+	if (value == 0)
+		return -EINVAL;
+	else if (value > 0)
 		value -= 1;
 
 	if (value != -1 && (value < 0 || value >= IWL_POWER_NUM))