spi: spi-axi: extend support for the `delay` field

The AXI SPI engine driver uses the `delay_usecs` field from `spi_transfer`
to configure delays, which the controller will execute.
This change extends the logic to also include the `delay` value, in case it
is used (instead if `delay_usecs`).

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20190926105147.7839-20-alexandru.ardelean@analog.com


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