Input: ad7877 - use new structure for SPI transfer delays

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 2b6d42495042 ("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.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>