rpc: Add typedef for the anonymous enum used for authentication methods

rpc: Add typedef for the anonymous enum used for authentication methods

Add typedef for the anonymous enum used for the authentication methods
and remove the default case. This allows the usage of the type in a
switch statement and taking advantage of the compilers feature to
detect uncovered cases.

Signed-off-by: Marc Hartmayer <mhartmay@linux.vnet.ibm.com>
Reviewed-by: Boris Fiuczynski <fiuczy@linux.vnet.ibm.com>
Reviewed-by: Stefan Zimmermann <stzi@linux.vnet.ibm.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>