accel/rocket: factor out code with find_core_for_dev in rocket_remove

accel/rocket: factor out code with find_core_for_dev in rocket_remove

There already is a function to return the offset of the core for a given
struct device, so let's reuse that function instead of reimplementing
the same logic.

There's one change in behavior when a struct device is passed which
doesn't match any core's. Before, we would continue through
rocket_remove() but now we exit early, to match what other callers of
find_core_for_dev() (rocket_device_runtime_resume/suspend()) are doing.
This however should never happen. Aside from that, no intended change in
behavior.

Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Tomeu Vizoso <tomeu@tomeuvizoso.net>
Signed-off-by: Tomeu Vizoso <tomeu@tomeuvizoso.net>
Link: https://patch.msgid.link/20251215-rocket-reuse-find-core-v1-1-be86a1d2734c@cherry.de