dmaengine: doc: fix warnings/issues of client.rst

This fixed some warnings/issues of client.rst.
 o Need a blank line for enumerated lists.
 o Do not create section in enumerated list.
 o Remove suffix ':' from section title.

Signed-off-by: Changbin Du <changbin.du@gmail.com>
Link: https://lore.kernel.org/r/20200204125115.12128-1-changbin.du@gmail.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>