cfg80211: remove division by size of sizeof(struct ieee80211_wmm_rule)

Pointer arithmetic already adjusts by the size of the struct,
so the sizeof() calculation is wrong. This is basically the
same as Colin King's patch for similar code in the iwlwifi
driver.

Fixes: 230ebaa1 ("cfg80211: read wmm rules from regulatory database")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>