Fix: event notifier: adapt read iterator state to poll expectations

Fix: event notifier: adapt read iterator state to poll expectations

When completing to read a subbuffer, ensure that the state of the
iterator is moved forward so the "put_subbuf" is performed before
returning to the user, so poll() will not return POLLIN when there
is actually no data available to read.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: Ia94b5dcb0c72d8548325b1004f214044f50fd191