staging: iio: update TODO

Since there are no uses of the old GPIO API, remove the item from
the TODO.
Signed-off-by: Rohit Sarkar <rohitsarkar5398@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/staging/iio/TODO

-2018-04-15
-
-All affected drivers:
-Convert all uses of the old GPIO API from <linux/gpio.h> to the
-GPIO descriptor API in <linux/gpio/consumer.h> and look up GPIO
-lines from device tree, ACPI or board files, board files should
-use <linux/gpio/machine.h>.
+2020-02-25
 
 
 ADI Drivers: