Fix annoying build warning when CONFIG_USB_SERIAL_GENERIC is undefined.

  drivers/usb/serial/generic.c:24: warning: `generic_probe' declared `static' but never defined
Signed-off-by: NDavid Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>