spi: core: remove 'delay_usecs' field from spi_transfer

spi: core: remove 'delay_usecs' field from spi_transfer

The 'delay' field in the spi_transfer struct is meant to replace the
'delay_usecs' field. However some cleanup was required to remove the
uses of 'delay_usecs'. Now that it's been cleaned up, we can remove it
from the kernel tree.

Signed-off-by: Alexandru Ardelean <aardelean@deviqon.com>
Link: https://lore.kernel.org/r/20210308145502.1075689-10-aardelean@deviqon.com
Signed-off-by: Mark Brown <broonie@kernel.org>