spi: spi-loopback-test: use new `delay` field

This change replaces the use of the `delay_usecs` field with the new
`delay` field. The code/test still uses micro-seconds, but they are now
configured and used via the `struct spi_delay` format of the `delay` field.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20190926105147.7839-13-alexandru.ardelean@analog.com
Signed-off-by: Mark Brown <broonie@kernel.org>