cfg80211: make wmm_rule part of the reg_rule structure

Make wmm_rule be part of the reg_rule structure. This simplifies the
code a lot at the cost of having bigger memory usage. However in most
cases we have only few reg_rule's and when we do have many like in
iwlwifi we do not save memory as it allocates a separate wmm_rule for
each channel anyway.

This also fixes a bug reported in various places where somewhere the
pointers were corrupted and we ended up doing a null-dereference.

Fixes: 230ebaa1 ("cfg80211: read wmm rules from regulatory database")
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
[rephrase commit message slightly]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>