accel/rocket: clear rdev on device init failure

In the Linux kernel, the following vulnerability has been resolved:

accel/rocket: clear rdev on device init failure

rocket_probe() stores the return value from rocket_device_init() in the
file-scoped rdev pointer before checking whether it is an error pointer.
If rocket_device_init() fails, rdev is left as an ERR_PTR.

The rdev pointer is also used as the marker for whether the shared rocket
device has already been initialized. A later core probe can therefore see
a non-NULL rdev, skip rocket_device_init(), and dereference the error
pointer when reading rdev->num_cores.

Save the error code and clear rdev before returning from the
rocket_device_init() failure path.