usb-serial: fix usb_serial_register bug when boot with nousb param

With "nousb" cmdline booting, built-in serial drivers (ie. airecable)
will trigger kernel oops.

Indeed, if nousb, usb_serial_init will failed, and the usb serial bus type
will not be registerd, then usb_serial_register call driver_register
which try to register the driver to a not registered bus.

Here add usb_disabled() check in usb_serial_register to fix it.

Signed-off-by: Dave Young <hidave.darkstar@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>