cfg80211: constify ieee80211_get_response_rate return

It's not modified so make it const with the eventual goal of moving
data to text for various static struct ieee80211_rate arrays.

Signed-off-by: Joe Perches <joe@perches.com>
Link: https://lore.kernel.org/r/8b210b5f5972e39eded269b35a1297cf824c4181.camel@perches.com


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