qcacld-3.0: Add error log in wow periodic pattern handler

WoW periodic transmit pattern handler has no error log if
given input format is wrong. Add error log message to indicate
input is wrong and suggest the correct input format.

Change-Id: Ie3a37c7e1a9b9fd8a974c68a5cc7456b1b622168
CRs-Fixed: 973006

core/hdd/src/wlan_hdd_debugfs.c

@@ -502,6 +502,7 @@ static ssize_t __wcnss_patterngen_write(struct file *file,
 	return count;
 
 failure:
+	hdd_err("Invalid input. Input format is: ptrn_idx duration pattern");
 	qdf_mem_free(cmd);
 	return -EINVAL;
 }