datapath-windows: Fix bug in OvsTcpGetWscale().

datapath-windows: Fix bug in OvsTcpGetWscale().

The userspace conntrack had a bug in tcp_wscale_get(), where the length
of an option would be read from the third octet of the option TLV
instead of the second.  This could cause an incorrect wscale value to
be returned, and it would at least impact performance.

Also use 'int' instead of 'unsigned' for 'len', since the value can be
negative.

CC: Sairam Venugopal <vsairam@vmware.com>
Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Acked-by: Sairam Venugopal <vsairam@vmware.com>