mac80211: always pick a basic rate to tx RTS/CTS for pre-HT rates

When the 1st rate control entry is a pre-HT rate we want to set
rts_cts_rate_idx "as the fastest basic rate that is not faster than the
data rate"(code comments).
But in case some bss allowed rate indexes are lower than the lowest bss
basic rate, if the rate control selects a rate among the formers for its
1st rate control entry, rts_cts_rate_idx remains 0 and is not a basic
rate index.
This commit sets rts_cts_rate_idx to the lowest bss basic rate index in
this situation.

Note that the code assumes that lowest indexes == lowest bitrates.

Signed-off-by: Karl Beldan <karl.beldan@rivierawaves.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>