mac80211_hwsim: allow creation of single-channel radios with chanctx

Add a new HWSIM_ATTR_USE_CHANCTX attribute to the
HWSIM_CMD_CREATE_RADIO command to allow the creation of radios with
one channel that use channel contexts.  If this attribute is not
present, the behaviour is the same as before (ie. single channel
radios don't use channel contexts and multi channel radios do).

Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>