mac80211: use short slot time in mesh for 5GHz

Use short slot time in 5GHz for mesh. The performance is
increased from 16.4Mbps to 23.4Mbps for two directly
connected mesh STAs operating in legacy rate using iperf
measurement. Almost similar to the results claimed in IBSS
mode.

Signed-off-by: Chun-Yeow Yeoh <yeohchunyeow@gmail.com>
[call ieee80211_get_sdata_band() only once]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>