spi: axi-spi-engine: remove struct spi_engine::msg

spi: axi-spi-engine: remove struct spi_engine::msg

In the AXI SPI Engine driver, the struct spi_engine::msg member was used
to keep track of the current message being processed. The SPI core is
already keeping track of this, so we don't need to duplicate the effort.

In most cases, we already have a pointer to the current message, so we
can pass it directly to the functions that need it. In the one case
where we don't have a pointer to the current message, we can get it
from struct spi_controller::cur_msg.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://lore.kernel.org/r/20231117-axi-spi-engine-series-1-v1-12-cc59db999b87@baylibre.com
Signed-off-by: Mark Brown <broonie@kernel.org>