p11kit: check the flags associated with the slot instead of flags associated with...

p11kit: check the flags associated with the slot instead of flags associated with the token

The logic around checking PKCS11 tokens (used by systemd-cryptenroll) contains a bug.
The code is checking the flags field of a pkcs11 token_info structure against a set of flags defined for the pkcs11 slot_info structure. This PR changes the check so that the correct structure's flag field is being checked.
(Reference to the PKCS#11 spec:http://docs.oasis-open.org/pkcs11/pkcs11-base/v2.40/os/pkcs11-base-v2.40-os.html, section CK_SLOT_INFO).