net: tcp: Store MSS in tcp header correctly

Use UNALIGNED_PUT() to store the MSS value into network packet
because the memory location cannot be guaranteed to be properly
aligned.

Change-Id: I77fd7a70ef45eedb657cac29457b0239b0a1d4c2
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>