iwlwifi: calculate active legacy rates per station

Not all stations are guaranteed to have the same
active (available) legacy rates, so calculate them
on rate control init instead of hard-coding them
based on our own available rates. I have no idea
why that was done here before.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>