iwlwifi: mvm: fix potential NULL-dereference in iwl_mvm_reorder()

We try to access sta before we check for IS_ERR_OR_NULL(), so we may
end up accessing a NULL pointer.  To prevent that, move the conversion
from sta to mvm_sta below the check.

Fixes: b915c101 ("iwlwifi: mvm: add reorder buffer per queue")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>