cfg80211: only pass sband to set_mandatory_flags_band()

The supported band structure contains the band is applies to
so no need to pass it separately. Also added a default case
to the switch for completeness. The current code base does not
call this function with NUM_NL80211_BANDS but kept that case
statement although default case would cover that.

Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>