net80211: define mask for ss_flags rather than using hardcoded 0xfff

scan state ss_flags in two places cut off the "internal" GOTPICK
options. Replace the hardcoded 0xfff with a defined mask.
Note that "internal" flags is confusing as we also supplement the
the 16bit by another 16bit of "internal flags" passed around but
comaparing to GOTPICK never stored to my understanding.

No functional change.

Sponsored by: The FreeBSD Foundation
Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D38832

(cherry picked from commit 3346164ce844f8995311c3988ab193bd6ddda0c9)