BLE Host - Allow MTU to be checked for unconn peer
BLE Host - Allow MTU to be checked for unconn peer

A previous commit (8ec1b8d9aa8b33a1a26e0d47841331870e97cd88) went a
little overboard in assuming a peer is connected.  Some code checks a
peer's MTU without knowing if the peer is connected.  A return value of
0 indicates the peer is not connected.

X-Original-Commit: 06c79c4be3d39acfeba09d843355411d6a28627f

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