staging: octeon-usb: check return value of platform_device_register_simple

the return value is a pointer having an error set. we have to check for
IS_ERR and return PTR_ERR when appropriate

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>