dw_dmac: Changing type of src_master and dest_master to u8.

src_master & dest_master don't required u32 as they have values limited to u8
only. Also their description is missing from doc style comment. This patch
fixes above mentioned issues.

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