spi: spidev: use new delay field for spi transfers

The `delay` field has type `struct spi_delay`.
This allows users to specify nano-second or clock-cycle delays (if needed).

Converting to use `delay` is straightforward: it's just assigning the
value to `delay.value` and hard-coding the `delay.unit` to
`SPI_DELAY_UNIT_USECS`.

This keeps the uapi for spidev un-changed. Changing it can be part of
another changeset and discussion.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20190926105147.7839-14-alexandru.ardelean@analog.com
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spidev.c

@@ -265,7 +265,8 @@ static int spidev_message(struct spidev_data *spidev,
 		k_tmp->tx_nbits = u_tmp->tx_nbits;
 		k_tmp->rx_nbits = u_tmp->rx_nbits;
 		k_tmp->bits_per_word = u_tmp->bits_per_word;
-		k_tmp->delay_usecs = u_tmp->delay_usecs;
+		k_tmp->delay.value = u_tmp->delay_usecs;
+		k_tmp->delay.unit = SPI_DELAY_UNIT_USECS;
 		k_tmp->speed_hz = u_tmp->speed_hz;
 		k_tmp->word_delay.value = u_tmp->word_delay_usecs;
 		k_tmp->word_delay.unit = SPI_DELAY_UNIT_USECS;