net80211: ieee80211_probereq_ie fix length calculation for hw scans

c338cf2c6d5eacdee813191d5976aa531d450ee7 split up ieee80211_probereq_ie().
For HW scans we usually do not want to add a SSID to the IEs.
During that split we allocate memory based on the length which will
always include the length of the SSID and only later we reduced the
length but never updated the value passed back to the caller.
Split the SSID handling up and reduce the length before malloc().
This not only makes us not over-allocate in these situatoins but also
fixes the length returned to the caller and with that usually directly
passed to firmware.

Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D30813