spi: axi-spi-engine: remove completed_id from driver state

In the AXI SPI Engine driver, the completed_id field in the driver
state is only used in one function and the value does not need to
persist between function calls. Therefore, it can be removed from the
driver state and made a local variable in the function where it is used.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://lore.kernel.org/r/20231117-axi-spi-engine-series-1-v1-11-cc59db999b87@baylibre.com


Signed-off-by: Mark Brown <broonie@kernel.org>