soundwire: bus: Don't filter slave alerts

It makes sense to have only a single point responsible for ensuring
that all currently pending IRQs are handled. The current code in
sdw_handle_slave_alerts confusingly splits this process in two.  This
code will loop until the asserted IRQs are cleared but it will only
handle IRQs that were already asserted when it was called. This
means the caller must also loop (either manually, or through its IRQ
mechanism) until the IRQs are all handled. It makes sense to either do
all the looping in sdw_handle_slave_alerts or do no looping there and
let the host controller repeatedly call it until things are handled.

There are realistically two sensible host controllers, those that
will generate an IRQ when the alert status changes and those
that will generate an IRQ continuously whilst the alert status
is high. The current code will work fine for the second of those
systems but not the first with out additional looping in the host
controller.  Removing the code that filters out new IRQs whilst
the handler is running enables both types of host controller to be
supported and simplifies the code. The code will still only loop up to
SDW_READ_INTR_CLEAR_RETRY times, so it shouldn't be possible for it to
get completely stuck handling IRQs forever, and if you are generating
IRQs faster than you can handle them you likely have bigger problems
anyway.

This fixes an issue on the Cadence SoundWire IP, which only generates
IRQs on an alert status change, where an alert which arrives whilst
another alert is being handled will never be handled and will block
all future alerts from being handled.

Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20230418140650.297279-1-ckeepax@opensource.cirrus.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>

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