cfg80211: validate beacon int as part of iface combinations

Remove the pointless checking against interface combinations in
the initial basic beacon interval validation, that currently isn't
taking into account radar detection or channels properly. Instead,
just validate the basic range there, and then delay real checking
to the interface combination validation that drivers must do.

This means that drivers wanting to use the beacon_int_min_gcd will
now have to pass the new_beacon_int when validating the AP/mesh
start.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>