Free cert usage before return

The ASN1 free will slowly leak memory for incorrect mutual auth
connections because if the certificate does not match the requirements
the function will return without freeing the usage string.

Tested: curl --cert client-cert.pem --key client-key.pem --cacert \
          CA-cert.pem https://${bmc}/redfish/v1/SessionService/Sessions

Change-Id: I4c335d3cd151187c7a10e7e668d1556c11389039
Signed-off-by: Vernon Mauery <vernon.mauery@linux.intel.com>