n_gsm: Fix length handling

If the mux is configured with a large mru/mtu the existing code gets the
byte ordering wrong for the header.
Signed-off-by: NKen Mills <ken.k.mills@intel.com>
Signed-off-by: NAlan Cox <alan@linux.intel.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

drivers/tty/n_gsm.c

@@ -716,8 +716,8 @@ static void __gsm_data_queue(struct gsm_dlci *dlci, struct gsm_msg *msg)
 		if (msg->len < 128)
 			*--dp = (msg->len << 1) | EA;
 		else {
-			*--dp = (msg->len >> 6) | EA;
-			*--dp = (msg->len & 127) << 1;
+			*--dp = ((msg->len & 127) << 1) | EA;
+			*--dp = (msg->len >> 6) & 0xfe;
 		}
 	}