net80211: correct IEEE80211_KEY_BITS

Not only are some of the bits missing, \4 and following were also
off-by-one. Correct that so we can use them instead of reading
hex numbers.

Sponsored by: The FreeBSD Foundation
Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D49366

(cherry picked from commit 0461600deaa227050fbff60274ba72ab3a407bcc)