Fix QAudioOutput::setVolume() limited 50% on 32-bit Windows

A signed 16 bit integer was being used to pack a normalised double into
half of a DWORD. It needed to be unsigned 16-bit to get the full range
of the Windows volume control.

Task-number: QTBUG-33160

Change-Id: Ic17f572a188401ee686c6e6af3984d52328ccda6
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>