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>