greybus: es1: release urb on error path

if error is return when submiting the urb, we need to make sure to release the
urb from the pool, or from the dinamicly allocated. As in it, factor out the free
code and create the free_urb function.

Signed-off-by: Rui Miguel Silva <rmfrfs@gmail.com>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>