nimble/ll: Fix calculation of scan rsp length
nimble/ll: Fix calculation of scan rsp length

This makes use of symbols to make calculations clear.

X-Original-Commit: a5f69610529dad2dd5827301edfe433597cf2eb8

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