spi: axi-spi-engine: remove use of ida for sync id

Profiling has shown that ida_alloc_range() accounts for about 10% of the
time spent in spi_sync() when using the AXI SPI Engine controller. This
call is used to create a unique id for each SPI message to match to an
IRQ when the message is complete.

Since the core SPI code serializes messages in a message queue, we can
only have one message in flight at a time, namely host->cur_msg. This
means that we can use a fixed value instead of a unique id for each
message since there can never be more than one message pending at a
time.

This patch removes the use of ida for the sync id and replaces it with a
constant value. This simplifies the driver and improves performance.

Reviewed-by: Nuno Sa <[email protected]>
Signed-off-by: David Lechner <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Mark Brown <[email protected]>

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