[PORTCLS] Fix audio stutter with official AC97 driver (#3225)

[PORTCLS] Fix audio stutter with official AC97 driver (#3225)

Replace call to AllocatedBufferSize(), with BufferSize().

Indeed (quoting https://docs.microsoft.com/en-us/windows-hardware/drivers/ddi/portcls/nf-portcls-idmachannel-buffersize ):

> The BufferSize() method returns the buffer size that was set by the previous call to IDmaChannel::SetBufferSize(). If SetBufferSize() has not been called since the IDmaChannel::AllocateBuffer() call, BufferSize returns the allocated buffer size. The DMA-channel object does not actually use this value internally. This value is maintained by the object to **allow its various clients to communicate the intended size of the buffer**.

And this is exactly what we want to do.