tty/n_gsm.c: get gsm->num after gsm_activate_mux

gsm->num is the index of gsm_mux[], it's invalid before calling gsm_activate_mux.
Signed-off-by: Nxinhui.pan <xinhuiX.pan@intel.com>
Acked-by: NAlan Cox <alan@linux.intel.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/n_gsm.c

@@ -2230,8 +2230,7 @@ static int gsmld_output(struct gsm_mux *gsm, u8 *data, int len)
 
 static int gsmld_attach_gsm(struct tty_struct *tty, struct gsm_mux *gsm)
 {
-	int ret, i;
-	int base = gsm->num << 6; /* Base for this MUX */
+	int ret, i, base;
 
 	gsm->tty = tty_kref_get(tty);
 	gsm->output = gsmld_output;
@@ -2241,6 +2240,7 @@ static int gsmld_attach_gsm(struct tty_struct *tty, struct gsm_mux *gsm)
 	else {
 		/* Don't register device 0 - this is the control channel and not
 		   a usable tty interface */
+		base = gsm->num << 6; /* Base for this MUX */
 		for (i = 1; i < NUM_DLCI; i++)
 			tty_register_device(gsm_tty_driver, base + i, NULL);
 	}