n_gsm: fix the wrong FCS handling

FCS could be GSM0_SOF, so will break state machine...

[This byte isn't quoted in any way so a SOF here doesn't imply an error
 occurred.]

Signed-off-by: Alek Du <alek.du@intel.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Cc: stable <stable@kernel.org> [3.0]

[Trivial but best backported once its in 3.1rc I think]
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/tty/n_gsm.c

@@ -1871,10 +1871,6 @@ static void gsm0_receive(struct gsm_mux *gsm, unsigned char c)
 		break;
 	case GSM_FCS:		/* FCS follows the packet */
 		gsm->received_fcs = c;
-		if (c == GSM0_SOF) {
-			gsm->state = GSM_SEARCH;
-			break;
-		}
 		gsm_queue(gsm);
 		gsm->state = GSM_SSOF;
 		break;