USB: mos7840.c: fix a check-after-dereference

This patch fixes an obvious check-after-dereference spotted by the
Coverity checker.
Signed-off-by: NAdrian Bunk <bunk@stusta.de>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

drivers/usb/serial/mos7840.c

@@ -2413,11 +2413,12 @@ static int mos7840_ioctl(struct usb_serial_port *port, struct file *file,
 	}
 
 	mos7840_port = mos7840_get_port_private(port);
-	tty = mos7840_port->port->tty;
 
 	if (mos7840_port == NULL)
 		return -1;
 
+	tty = mos7840_port->port->tty;
+
 	dbg("%s - port %d, cmd = 0x%x", __FUNCTION__, port->number, cmd);
 
 	switch (cmd) {