spi: axi-spi-engine: simplify driver data allocation

This simplifies the private data allocation in the AXI SPI Engine driver
by making use of the feature built into the spi_alloc_host() function
instead of doing it manually.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://lore.kernel.org/r/20231117-axi-spi-engine-series-1-v1-3-cc59db999b87@baylibre.com


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