iio: adc: ti_am335x_adc: Add a unit to the timeout delay

The lack of unit in the macro name kind of tricked me when I was
troubleshooting an issue. Physical constants should always get a unit.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20211015081506.933180-44-miquel.raynal@bootlin.com