accel/rocket: Fix usages of kfree() and sizeof()

[Markus Elfring]

> I had considered splitting it into two patches, but the changes were so
> minor that I figured it wasn't worth it. Please let me know if you
> prefer separate patches and I'll gladly prep another revision.

You can distinguish mentioned change possibilities better also according to
their severity, can't you?

Regards,
Markus