cfg80211: use only 1Mbps for basic rates in mesh

Mesh used to use the mandatory rates as basic rates, but we got
the calculation of mandatory rates wrong until some time ago.
Fix this this broke interoperability with older versions since
now more basic rates are required, and thus the MBSS isn't the
same and the network stops working.

Fix this by simply using only 1Mbps as the basic rate in 2.4GHz.
Since the changed mandatory rates only affected 2.4GHz, this is
all we need to make it work again.

Reported-and-tested-by: Matthias Schiffer <mschiffer@universe-factory.net>
Fixes: 1bd773c0 ("wireless: set correct mandatory rate flags")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>