staging: octeon-usb: call device_unregister when platform_device_register_simple fails

device_register is called before platform_device_register_simple gets called.
unregister and reset the octeon_usb_registered variable

Cc: Aaro Koskinen <aaro.koskinen@iki.fi>
Cc: David Daney <ddaney.cavm@gmail.com>
Signed-off-by: Devendra Naga <devendra.aaru@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>