Use an enum for EAP SM requests

Use an enum for EAP SM requests

Control requests will be extended for non-EAP uses later, so it makes
sense to have them be generic. Furthermore, having them defined as an
enum is easier for processing internally, and more generic for control
interfaces that may not use field names. The public ctrl_req_type /
field_name conversion function will be used later by the D-Bus control
interface too.

Signed-off-by: Dan Williams <dcbw@redhat.com>