wifi: cfg80211: make cfg80211_auth_request::key_idx signed

We might assign -1 to it in some cases when key is NULL,
which means the key_idx isn't used but can lead to a
warning from static checkers such as smatch.

Make the struct member signed simply to avoid that, we
only need a range of -1..3 anyway.

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