cfg80211: allow leaving MU-MIMO monitor configuration unchanged

When changing monitor parameters, not setting the MU-MIMO attributes
should mean that they're not changed - it's documented that to turn
the feature off it's necessary to set all-zero group membership and
an invalid follow-address. This isn't implemented.

Fix this by making the parameters pointers, stop reusing the macaddr
struct member, and documenting that NULL pointers mean unchanged.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>