nimble/ll: Fix TxAdd field for ADV_EXT_IND
nimble/ll: Fix TxAdd field for ADV_EXT_IND

We never include AdvA on primary channel so TxAdd should be set to 0 as
it's RFU in such case.

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