char: lp: use first unused lp number while registering

When the parallel port is usb based and the lp attaches to it based on
LP_PARPORT_AUTO, we do get /dev/lp0 and when we remove the usb device
/dev/lp0 is unregistered. But if we now reconnect the usb device we get
/dev/lp1, another disconnection and reconnection and we get /dev/lp2.

Use the port number array to find the first unused lp number and use
that to register the lp device with the parallel port.
Signed-off-by: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/char/lp.c

@@ -958,7 +958,11 @@ static void lp_attach(struct parport *port)
 			printk(KERN_INFO "lp: ignoring parallel port (max. %d)\n",LP_NO);
 			return;
 		}
-		if (!lp_register(lp_count, port))
+		for (i = 0; i < LP_NO; i++)
+			if (port_num[i] == -1)
+				break;
+
+		if (!lp_register(i, port))
 			lp_count++;
 		break;