tcp: micro-optimize SYN-cookie expansion

Only compute wscale when it is actually used. While there, change the
type of wscale to u_int as suggested by glebius.
No functional change intended.

Reviewed by: glebius, rscheff (older version)
MFC after: 1 week
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D52296