mac80211: allow SMPS requests only in client mode

The code currently allows this for AP mode, but then ignores
it. Clarify that since the spec doesn't allow it in AP mode.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20210618133832.0465f8bcbe32.Iba39fc559ecfa887be00a5f3beabd881e5c86e54@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>