async_tx: deprecate broken support for channel switching

Back in 2011, Russell pointed out that the "async_tx channel switch"
capability was violating expectations of the dma mapping api [1]. At the
time the existing uses were reviewed as still usable, but that longer
term we needed a rework of the raid offload implementation. While some
of the framework for a fixed implementation was introduced in 2012 [2],
the wider rewrite never materialized.

There continues to be interest in raid offload with new dma/raid engine
drivers being submitted. Those drivers must not build on top of the
broken channel switching capability.

Prevent async_tx from using an offload engine if the channel switching
capability is enabled. This still allows the engine to be used for other
purposes, but the broken way async_tx uses these engines for raid will
be disabled. For configurations where this causes a performance
regression the only solution is to start the work of eliminating the
async_tx api and moving channel management into the raid code directly
where it can manage marshalling an operation stream between multiple dma
channels.

[1]: http://lists.infradead.org/pipermail/linux-arm-kernel/2011-January/036753.html
[2]: https://lkml.org/lkml/2012/12/6/71

Cc: Anatolij Gustschin <agust@denx.de>
Cc: Anup Patel <anup.patel@broadcom.com>
Cc: Rameshwar Prasad Sahu <rsahu@apm.com>
Cc: Saeed Bishara <saeed.bishara@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reported-by: Russell King <linux@armlinux.org.uk>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>

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