Urgent: S.BUS input on 4.0.1 is stuttering often 100-300ms, up to 500ms

[tridge]

Here is a PR that fixes the issue:

Fixed RC input timing jitter in IOMCU

Here is a binary for CubeBlack for testing:
http://uav.tridgell.net/tmp/copter-CubeBlack-sbus-fix.apj
Please test!