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.

andrzej-kaczmarek/apache-mynewt-core,mlaz/mynewt-core,andrzej-kaczmarek/apache-mynewt-core,andrzej-kaczmarek/apache-mynewt-core,IMGJulian/incubator-mynewt-core,mlaz/mynewt-core,andrzej-kaczmarek/incubator-mynewt-core,andrzej-kaczmarek/incubator-mynewt-core,andrzej-kaczmarek/incubator-mynewt-core,andrzej-kaczmarek/incubator-mynewt-core,mlaz/mynewt-core,IMGJulian/incubator-mynewt-core,andrzej-kaczmarek/incubator-mynewt-core,IMGJulian/incubator-mynewt-core,mlaz/mynewt-core,IMGJulian/incubator-mynewt-core,IMGJulian/incubator-mynewt-core,andrzej-kaczmarek/apache-mynewt-core,mlaz/mynewt-core