USB: fix usb-serial/generic build warning

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: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>