greybus: operation: drop OOM-response FIXME

Drop FIXME about sending responses in OOM situations.

If we fail to allocate an operation for an incoming request, we have
bigger problems than to worry about sending a response.

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