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
MFC after: 3 days
Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D38832