tty/n_gsm: fix a bug in gsm_dlci_data_output (adaption = 2 case)

in adaption=2 case we should put 1 or 2 byte with modem status bits
at the beginning of a buffer pointed by "dp". n_gsm use 1 byte case,
so it allocate a buffer of len + 1 size. As result we should:
  * put 1 byte of modem status bits
  * increase data pointer
  * put "len" bytes of data
but actually we have:
  * increase first byte with the value of modem status bits
  * decrease "len"
  * put orig_len - 1 bytes of data starting from the buffer beggining
This is evidently wrong.

Signed-off-by: Mikhail Kshevetskiy <mikhail.kshevetskiy@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/tty/n_gsm.c

@@ -833,8 +833,7 @@ static int gsm_dlci_data_output(struct gsm_mux *gsm, struct gsm_dlci *dlci)
 		break;
 	case 2:	/* Unstructed with modem bits. Always one byte as we never
 		   send inline break data */
-		*dp += gsm_encode_modem(dlci);
-		len--;
+		*dp++ = gsm_encode_modem(dlci);
 		break;
 	}
 	WARN_ON(kfifo_out_locked(dlci->fifo, dp , len, &dlci->lock) != len);