iwlwifi: mvm: Allow multicast data frames only when associated

The MAC context configuration always allowed multicast data frames
to pass to the driver for all MAC context types, and in the
case of station MAC context both when associated and when not
associated.

One of the outcomes of this configuration is having the FW forward
encrypted multicast frames to the driver with Rx status indicating
that the frame was not decrypted (as expected, since no keys were
configured yet) which in turn results with unnecessary error
messages.

Change this behavior to allow multicast data frames only when they
are actually expected, e.g., station MAC context is associated etc.

Signed-off-by: Ilan Peer <ilan.peer@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>