greybus: Loopback_test: use poll instead of inotify

Inotify does not handle sysfs events, so use poll instead.

The loopback kernel driver will send a notification when the test is
complete. So, open a poll file descriptor for every enabled device,
and after starting the test, wait for an event from each device.
After all events are received, read the total number of iterations
and make sure the test is complete.

Also, add missing stdint header which was included in inotify.

Signed-off-by: Axel Haslam <ahaslam@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>