Input: ad7877 - 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 bebcfd27 ("spi: introduce `delay` field for
`spi_transfer` + spi_transfer_delay_exec()")
Signed-off-by: Sergiu Cuciurean <sergiu.cuciurean@analog.com>
Link: https://lore.kernel.org/r/20200228104508.15564-1-sergiu.cuciurean@analog.comSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/touchscreen/ad7877.c

@@ -281,12 +281,14 @@ static int ad7877_read_adc(struct spi_device *spi, unsigned command)
 
 	req->xfer[1].tx_buf = &req->ref_on;
 	req->xfer[1].len = 2;
-	req->xfer[1].delay_usecs = ts->vref_delay_usecs;
+	req->xfer[1].delay.value = ts->vref_delay_usecs;
+	req->xfer[1].delay.unit = SPI_DELAY_UNIT_USECS;
 	req->xfer[1].cs_change = 1;
 
 	req->xfer[2].tx_buf = &req->command;
 	req->xfer[2].len = 2;
-	req->xfer[2].delay_usecs = ts->vref_delay_usecs;
+	req->xfer[2].delay.value = ts->vref_delay_usecs;
+	req->xfer[2].delay.unit = SPI_DELAY_UNIT_USECS;
 	req->xfer[2].cs_change = 1;
 
 	req->xfer[3].rx_buf = &req->sample;