iwlwifi: mvm: Avoid dereferencing sta if it was already flushed

Be a little bit more careful when dereferencing sta on key removal,
As it might already get flushed on other thread.

Signed-off-by: Avri Altman <avri.altman@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>