greybus: update svc_msg_header fields and users to match spec

greybus: update svc_msg_header fields and users to match spec

The Greybus spec has been updated to clarify some of the original
intent of the SVC message definition. The svc_msg_header was:

struct svc_msg_header {
	__u8 function;
	__u8 message_type;
...
}

and is now

struct svc_msg_header {
	__u8 function_id;
	__u8 message_type;
...
}

to match the spec. The function_id carries enum svc_function_id values
and message_type is now clarified to be a session layer level field
that is simply "data" or "error".

Change all references of function type to function id. For now, don't
parse the message_type field but add the two allowable svc_msg_type enums.

Signed-off-by: Matt Porter <mporter@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>