ieee80211: fix -Wcast-qual warnings

When enabling -Wcast-qual e.g. via W=3, we get a lot of
warnings from this file, whenever it's included. Since
the fixes are simple, just do that.

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.20220202104617.79ec4a4bab29.I8177a0c79d656c552e22c88931d8da06f2977896@changeid


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