The majority of drivers in drivers/dma/ will never establish cross
channel operation chains and do not need the extra overhead in struct
dma_async_tx_descriptor. Make channel switching opt-in by default.
Cc: Anatolij Gustschin <agust@denx.de> Cc: Ira Snyder <iws@ovro.caltech.edu> Cc: Linus Walleij <linus.walleij@stericsson.com> Cc: Saeed Bishara <saeed@marvell.com> Signed-off-by: Dan Williams <dan.j.williams@intel.com>