spi: axi-spi-engine: don't repeat mode config for offload

spi: axi-spi-engine: don't repeat mode config for offload

Add an optimization to avoid repeating the config instruction in each
SPI message when using SPI offloading. Instead, the instruction is
run once when the SPI offload trigger is enabled.

This is done to allow higher sample rates for ADCs using this SPI
controller.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20250428-adi-main-v1-2-4b8a1b88a212@baylibre.com
Signed-off-by: Mark Brown <broonie@kernel.org>