spi: axi-spi-engine: don't emit XFER_BITS for empty xfer

This adds a check on xfer->len to avoid emitting an XFER_BITS
instruction for empty transfers in the AXI SPI Engine driver. This
avoids unnecessary delays caused by executing an instruction that has
no effect on the actual SPI transfer.
Signed-off-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Link: https://patch.msgid.link/20240723-spi-axi-spi-engine-opt-bpw-v1-1-2625ba4c4387@baylibre.comSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-axi-spi-engine.c

@@ -258,7 +258,7 @@ static void spi_engine_compile_message(struct spi_message *msg, bool dry,
 					clk_div - 1));
 		}
 
-		if (bits_per_word != xfer->bits_per_word) {
+		if (bits_per_word != xfer->bits_per_word && xfer->len) {
 			bits_per_word = xfer->bits_per_word;
 			spi_engine_program_add_cmd(p, dry,
 				SPI_ENGINE_CMD_WRITE(SPI_ENGINE_CMD_REG_XFER_BITS,