greybus: es2: No need to check before freeing an urb

usb_kill_urb() and usb_free_urb() can be called with NULL pointers, so
no need to check before calling them.

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