mac80211: re-organize the rx rate calculation logic

Currently the logic to fill a struct rate_info with
a STA's last RX rate is accessible only in the cfg.c.
As the RX rate calculation might be needed elsewhere,
split this out into a separate function.

Signed-off-by: Saravana <saravanad@posedge.com>
[fix various whitespace issues, reword commit log]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>