dmaengine/dw_dmac: Divide one sg to many desc, if sg len is greater than DWC_MAX_COUNT

If len passed in sg for slave_sg transfers is greater than DWC_MAX_COUNT, then
driver programmes controller incorrectly.  This patch adds code to handle this
situation by allocation more than one desc for same sg.

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