nimble/dtm: Allow for being late on RX start timer
nimble/dtm: Allow for being late on RX start timer

So far we assert when being late or starting RX timer. Now we allow to
be late as it is not important for dtm code. RX is still on after it.

X-Original-Commit: dc600a051f11e3c9f31e93fdd45e63c659c3c8c4

apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble