mac80211: exclude multicast frames from BA accounting

Since multicast aggregation is not currently supported by
mac80211, we can safely ignore them when reordering
aggregate MPDUs.

This fixes a bug where the expected sequence number might
be reset after processing a multicast frame from a STA
with an established aggregation session. The expected
sequence number would then be incorrect and all aggregated
frames until the sequence number rolled over would be
dropped.

Signed-off-by: Thomas Pedersen <thomas@cozybit.com>