spi: axi-spi-engine: optimize bits_per_word for offload

spi: axi-spi-engine: optimize bits_per_word for offload

Add an optimization to avoid repeating bits_per_word instructions in
each message when using SPI offload. This only applies when all data
xfers in a message have the same bits_per_word. In this case, we can
execute the instruction that sets bits_per_word when the offload trigger
is enabled. This is useful e.g. for obtaining higher sample rates on
ADCs since each message takes less time to execute.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20250428-adi-main-v1-3-4b8a1b88a212@baylibre.com
Signed-off-by: Mark Brown <broonie@kernel.org>