Recent TCP socket change bugfix

Pull request #899 introduced a small change that looked innocuous enough, but turned out to completely break the `tcp-socket` module. The horrible `uip_mss()` macro uses the "current" TCP connection to compute the MSS, but when registering a socket this typically isn't set, so the TCP socket will be created with a minimum segment size of 0, causing the socket to become completely unable to send any data.

This pull request should fix it.