iio: adc: ad7793: replace usleep_range() with fsleep()

The AD7792/AD7793 datasheet (Rev. B, page 25, RESET section)
says: "When a reset is initiated, the user must allow a period
of 500 us before accessing any of the on-chip registers."

Use fsleep(500) instead of usleep_range(500, 2000). The 500 us
minimum stays the same; fsleep() picks the upper slack itself
(about +25% on a default config -- narrower than the original
2000 us).

Add a code comment with the datasheet reference so the "why"
of the wait is visible at the call site.

Signed-off-by: Stepan Ionichev <sozdayvek@gmail.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

Author: root3315

drivers/iio/adc/ad7793.c

@@ -268,7 +268,12 @@ static int ad7793_setup(struct iio_dev *indio_dev,
 	ret = ad_sd_reset(&st->sd);
 	if (ret < 0)
 		goto out;
-	usleep_range(500, 2000); /* Wait for at least 500us */
+
+	/*
+	 * Per AD7792/AD7793 datasheet (Rev. B, page 25, RESET section),
+	 * allow 500 us after a reset before accessing on-chip registers.
+	 */
+	fsleep(500);
 
 	/* write/read test for device presence */
 	ret = ad_sd_read_reg(&st->sd, AD7793_REG_ID, 1, &id);