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 <[e-mail]>
Signed-off-by: Alan Cox <[e-mail]>
Cc: stable <[e-mail]> [3.0]

[Trivial but best backported once its in 3.1rc I think]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/n_gsm.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;