net: tcp: Fix uninitialized garbage in TCP urgent field

Per spec, this is required to be zero when URG is unset, but we were
never initializing it.  It seems benign AFAICT, but produces warnings
in wireshark.

Change-Id: Ie4e28070e32767aabc60c63bb85287c1215ebb0d
Signed-off-by: Andy Ross <andrew.j.ross@intel.com>
Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>