wireless: set correct mandatory rate flags

According to IEEE Std 802.11-2016 (16.2.3.4 Long PHY SIGNAL field) all of
the following rates are mandatory for a HR/DSSS PHY: 1 Mb/s, 2 Mb/s,
5.5 Mb/s and 11 Mb/s. Set IEEE80211_RATE_MANDATORY_B flag for all of these
instead of just 1 Mb/s to correctly reflect this.

Signed-off-by: Richard Schütz <rschuetz@uni-koblenz.de>
[johannes: use switch statement]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>