iwlwifi: mvm: remove division by size of sizeof(struct ieee80211_wmm_rule)

The subtraction of two struct ieee80211_wmm_rule pointers leaves a result
that is automatically scaled down by the size of the size of pointed-to
type, hence the division by sizeof(struct ieee80211_wmm_rule) is
bogus and should be removed.

Detected by CoverityScan, CID#1467777 ("Extra sizeof expression")

Fixes: 77e30e10 ("iwlwifi: mvm: query regdb for wmm rule if needed")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>