BLE Host - Cleanup att_svr rx prep / exec write.
BLE Host - Cleanup att_svr rx prep / exec write.

While debugging an mbuf leak, I noticed this code was much more
complicated than it needs to be.

X-Original-Commit: 79922ef3b2f0f52721a23f4a62d0f9a34f39737d

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