mac80211: allow lowest basic rate for unicast management for mesh

Allow lowest basic rate to be used for unicast management frame in
mesh. Otherwise, the lowest supported rate is used for unicast
management frame, such as 1Mbps for 2.4GHz and 6Mbps for 5GHz. Rename
the rc_send_low_broadcast to re_send_low_basicrate since now it is
also applied to unicast management frame in mesh.

Signed-off-by: Chun-Yeow Yeoh <yeohchunyeow@cozybit.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>