{nl,mac,cfg}80211: Allow user to configure basic rates for mesh

Currently mesh uses mandatory rates as the default basic rates. Allow basic
rates to be configured during mesh join. Basic rates are applied only if
channel is also provided with mesh join command.

Signed-off-by: Ashok Nagarajan <ashok@cozybit.com>
[some whitespace fixes, refuse basic rates w/o channel]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>