drivers: dma: sdma: Fix noise issue with pause/resume

Each time we configure an SDMA channel we also compute the total
allocated DMA buffer length but we assume is initialized with zero.

This is true each time a channel is requested. But if there are
multiple calls to configure without releasing the channel the buffer
length is not correctly computed.

So, we need to initialize it with zero each time we reconfigure the dma
channel.

Signed-off-by: Daniel Baluta <daniel.baluta@nxp.com>