mac80211: airtime: avoid variable shadowing

This isn't very dangerous, since the outer 'rate' variable
isn't even a pointer, but it's still confusing, so use a
different variable inside.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20220202104617.8e9b2bfaa0f5.I41c53f754eef28206d04dafc7263ccb99b63d490@changeid


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