tlssession: Rx buffer should not use the MTU as the max size

The MTU is not symetric, so the other device can send messages
larger than the local MTU (or don't use any MTU). The Rx buffer
should use the maximum size of a UDP packet (64k) instead the MTU.

Change-Id: I1ef6f17b041b25dc6a364c237527efe84927e6db
Gitlab: #14