USB: serial: remove bizarre generic_serial probe function

I can't remember why I wrote it like this many many years ago, but it's
not needed at all, let's rely on the usb-serial core for this function,
especially as it is being overridden by it anyway.

This lets us make usb_serial_probe() a static function, which it should
be.

Cc: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/serial/usb-serial.c

@@ -699,7 +699,7 @@ static const struct tty_port_operations serial_port_ops = {
 	.shutdown = serial_down,
 };
 
-int usb_serial_probe(struct usb_interface *interface,
+static int usb_serial_probe(struct usb_interface *interface,
 			       const struct usb_device_id *id)
 {
 	struct usb_device *dev = interface_to_usbdev(interface);