api: dma: Fix comments for struct dma_config

The comments refered to 'config' and 'config_size' as though they
were named members of the struct, which they are not, and so is a little
confusing. Delete these comments and also correctly align the text for
size and burst members.

Change-Id: Iae14c76940268b8e7d72b117c8aea5a204b3da34
Signed-off-by: Jon Medhurst <tixy@linaro.org>