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.com


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