dmaengine/dw_dmac: Don't use magic number for total number of channels

Total number of channels is passed in pdata->nr_channels variable, thus we must
not use magic number '7' for total number of channels.

Signed-off-by: Viresh Kumar <viresh.kumar@st.com>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>