nimble/ll: Fix calculation of adv pdu length
nimble/ll: Fix calculation of adv pdu length

This makes use of symbols to make calculations clear.

X-Original-Commit: 6fc22367e19e4c1ae12958e54cafd0b78372f707

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