mesh: Fix resetting heartbeat subscription expiry properly
mesh: Fix resetting heartbeat subscription expiry properly

Set the value clearly to 0 instead of letting the old expiry time stay
around.

X-Original-Commit: 37f270cf792f64eef9a4af2c02f09a07d72b3149

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