nl80211: allow client-only BIGTK support

The current NL80211_EXT_FEATURE_BEACON_PROTECTION feature flag
requires both AP and client support, add a new one called
NL80211_EXT_FEATURE_BEACON_PROTECTION_CLIENT that enables only
support in client (and P2P-client) modes.

Link: https://lore.kernel.org/r/20200420140559.6ba704053a5a.Ifeb869fb0b48e52fe0cb9c15572b93ac8a924f8d@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>