selftests/tls: fix selftests after adding ChaCha20-Poly1305

TLS selftests where broken because of wrong variable types used.
Fix it by changing u16 -> uint16_t

Fixes: 4f336e88

 ("selftests/tls: add CHACHA20-POLY1305 to tls selftests")
Reported-by: kernel test robot <oliver.sang@intel.com>
Signed-off-by: Vadim Fedorenko <vfedorenko@novek.ru>
Link: https://lore.kernel.org/r/1610141865-7142-1-git-send-email-vfedorenko@novek.ru
Signed-off-by: Jakub Kicinski <kuba@kernel.org>