Windows: Fix no longer receive data after EV_TXEMPTY triggered for CDC ACM device

A previous patch

* SHA1 ID: 2bcac223e64599eb74889975053f6547000b13cb

introduced a regression: after triggering of EV_TXEMPTY a reception of
data is stopped. The reason for this was an immediate return from the
handler processCompletionRoutine().

The solution - do not do an immediate return and let to call method
startAsyncRead().

Thanks to Dan

Task-number: QTPLAYGROUND-22
Change-Id: I1a6b358e6bbaba94527dc9db98a2ab046a8c6f05
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>
Reviewed-by: Laszlo Papp <lpapp@kde.org>