davinci_cpdma: Fix channel number written to teardown registers

chan->chan_num is 0..CPDMA_MAX_CHANNELS-1 for tx channels and
CPDMA_MAX_CHANNELS..2*CPDMA_MAX_CHANNELS-1 for rx channels. However,
the rx and tx teardown registers expect zero based channel numbering.

Since the upper bits of the registers are reserved, the teardown also
worked before, this patch is cleanup only.

Signed-off-by: Christian Riesch <christian.riesch@omicron.at>
Signed-off-by: David S. Miller <davem@davemloft.net>