greybus: es2: separate urb allocation and submission

Refactor in-urb submission, and make sure to separate allocation and
submission.

Eventually we'll be submitting the urbs at cport enable rather than at
probe, and this is also needed for the driver-model rework.

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