mac80211_hwsim: fix overwriting of if_combination

Moved setting if_combination.num_different_channels/radar_detect_widths
into an else after use_chanctx. In the case of use_chanctx, these two
settings were getting overwritten.

Signed-off-by: James Prestwood <james.prestwood@linux.intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>