WPS: Truncate variable length string attributes to maximum length

WPS: Truncate variable length string attributes to maximum length

This enforces variable length strings Manufacturer, Model Name, Model
Number, and Serial Number to be within the maximum length defined in the
WSC specification. While none of the existing users for these within
hostapd/wpa_supplicant had problems with longer strings, it is good to
ensure the strings are not longer to avoid potential issues at higher
layer components.

Signed-off-by: Jouni Malinen <j@w1.fi>