iwlwifi: mvm: don't crash on invalid rate w/o STA

If we get to the WARN_ONCE(..., "Got a HT rate (...)", ...)
here with a NULL sta, then we crash because mvmsta is bad
and we try to dereference it. Fix that by printing -1 as the
state if no station was given.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Fixes: 6761a718 ("iwlwifi: mvm: add explicit check for non-data frames in get Tx rate")
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/iwlwifi.20211203140410.1a1541d7dcb5.I606c746e11447fe168cf046376b70b04e278c3b4@changeid