greybus: es2: remove bulk_out array

We only care about one bulk out endpoint, the first one, so remove
the pretense of keeping an array of these things.  Just grab the first
one in the list and run away!

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