iio: adc: mcp320x: Use new structure for SPI transfer delays

In a recent change to the SPI subsystem [1], a new `delay` struct was added
to replace the `delay_usecs`. This change replaces the current
`delay_usecs` with `delay` for this driver.

The `spi_transfer_delay_exec()` function [in the SPI framework] makes sure
that both `delay_usecs` & `delay` are used (in this order to preserve
backwards compatibility).

[1] commit bebcfd272d ("spi: introduce `delay` field for
`spi_transfer` + spi_transfer_delay_exec()")

Signed-off-by: Sergiu Cuciurean <sergiu.cuciurean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/mcp320x.c

@@ -421,7 +421,8 @@ static int mcp320x_probe(struct spi_device *spi)
 			adc->transfer[1].len++;
 
 		/* conversions are started by asserting CS pin for 8 usec */
-		adc->start_conv_transfer.delay_usecs = 8;
+		adc->start_conv_transfer.delay.value = 8;
+		adc->start_conv_transfer.delay.unit = SPI_DELAY_UNIT_USECS;
 		spi_message_init_with_transfers(&adc->start_conv_msg,
 						&adc->start_conv_transfer, 1);