mac80211: minstrel_ht: use bitfields to encode rate indexes

Get rid of a lot of divisions and modulo operations
Reduces code size and improves performance

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Link: https://lore.kernel.org/r/20210127055735.78599-1-nbd@nbd.name


Signed-off-by: Johannes Berg <johannes.berg@intel.com>