dmaengine: doc: fix bullet list formatting

The bullet list documenting the 'struct dma_device' fields has several
nesting errors, making it render improperly. It also has incoherent
formatting: some fields have a description in the same bullet, some in
a sub-bullet.

Fix both to have a correct and coherent formatting.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>