greybus: operation: fix definition of the invalid type

The invalid request type has been redefined as 0x7f.

Also remove the redundant redefinition of the invalid type from the
operation header.

Note that operation type 0x00 has been repurposed for the new generic
ping operation, which will be used to implement proper connection tear
down.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>