cfg80211: apply same mandatory rate flags for 5GHz and 6GHz

For the new 6GHz band the same rules apply for mandatory rates so
add it to set_mandatory_flags_band() function.

Reviewed-by: Pieter-Paul Giesberts <pieter-paul.giesberts@broadcom.com>
Reviewed-by: Leon Zegers <leon.zegers@broadcom.com>
Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Link: https://lore.kernel.org/r/1564745465-21234-9-git-send-email-arend.vanspriel@broadcom.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>