spi: tegra20-sflash: use to new `spi_transfer_delay_exec`

spi: tegra20-sflash: use to new `spi_transfer_delay_exec`

This conversion to the spi_transfer_delay_exec() helper is not
straightforward, as it seems that when a delay is present, the controller
issues a command, and then a delay is followed.

This change adds support for the new `delay` field which is of type
`spi_delay` and keeps backwards compatibility with the old `delay_usecs`
field.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20190926105147.7839-12-alexandru.ardelean@analog.com
Signed-off-by: Mark Brown <broonie@kernel.org>