Clear AsyncFd readiness in read_event

[RoastVeg]

@seimonw correctly identified in #6 that tokio-gpiod fails in repeated calls to read_event with EWOULDBLOCK/EAGAIN, but mistakenly assumes it's due to NONBLOCK having been set. This is actually due to the fact that the readiness flag in AsyncFd is not cleared after the first call to readable.

The easiest way to solve the problem is by following the example given in the AsyncFd docs

[nsabovic]

Just a ping to see if we can have this merged?

I've verified that it works in my code, but it can also be tested with the CLI—without this commit tgpio mon fails after the first state change. With this change the CLI works and doesn't have spurious wakeups.