mac80211_hwsim: fix NUM_BANDS usage

Due to the recent change of NUM_BANDS from 2 to 3 hwsim
broke. Fix the code by using the right constant but don't
support bands other than 2.4 and 5 GHz.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>