cfg80211: return immediately if num reg rules > NL80211_MAX_SUPP_REG_RULES

This has no functional change except we save a kfree(rd) and
allows us to clean this code up a bit after this. We do
avoid an unnecessary kfree(NULL) but calling that was OK too.

Cc: stable@kernel.org
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>