spi: docs: update info about 'delay_usecs'

The 'delay_usecs' field is no longer present on the spi_transfer struct.
This change updates the doc to mention the usage of the (relatively) new
'delay' field.

Signed-off-by: Alexandru Ardelean <aardelean@deviqon.com>
Link: https://lore.kernel.org/r/20210308145502.1075689-11-aardelean@deviqon.com


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