UPSTREAM: 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>

Bug: 253289327
Change-Id: Iaaa6e53137530af1c5364de880c7e0947abd19bc
(cherry picked from commit 23cc6d8c)
Signed-off-by: Kiran Kumar Lokere <quic_klokere@quicinc.com>