pf: correctly reset max_win if the SYN-ACK lacks a wscale option.

pf was setting max_win to 0 and discarded retransmitted SYN-ACK segments without
wscale if the original SYN contained a wscale option. with gerhard@, ok
henning@

Obtained From: OpenBSD
Sponsored by: Rubicon Communications, LLC ("Netgate")