net: tcp: Remove multiple checks on nbuf protocol family

Protocol family is checked in prepare_segment() and in same function
it's again verified by finalize_segment(). So remove the double checking
in finalize_segment().

Change-Id: I17123ab8741d017d7e3ff1ef3fb07371b0d4aa66
Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>