net80211: add an IEEE80211_IS_PROTECTED() macro

Summary: This returns whether the given 802.11 frame has the protected bit set.

Test Plan:

• tested in AP/STA mode
• STA mode - local athp/ath10k driver
• AP mode - in tree ath driver

Subscribers: imp, melifaro, glebius
Reviewed by: bz
Approved by: bz

Differential Revision: https://reviews.freebsd.org/D36183