iio: ltc2497: Fix reading conversion results

After the result of the previous conversion is read the chip
automatically starts a new conversion and doesn't accept new i2c
transfers until this conversion is completed which makes the function
return failure.

So add an early return iff the programming of the new address isn't
needed. Note this will not fix the problem in general, but all cases
that are currently used. Once this changes we get the failure back, but
this can be addressed when the need arises.

Fixes: 69548b7c ("iio: adc: ltc2497: split protocol independent part in a separate module ")
Reported-by: Meng Li <Meng.Li@windriver.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Tested-by: Denys Zagorui <dzagorui@cisco.com>
Cc: <Stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20220815091647.1523532-1-dzagorui@cisco.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/ltc2497.c

@@ -41,6 +41,19 @@ static int ltc2497_result_and_measure(struct ltc2497core_driverdata *ddata,
 		}
 
 		*val = (be32_to_cpu(st->buf) >> 14) - (1 << 17);
+
+		/*
+		 * The part started a new conversion at the end of the above i2c
+		 * transfer, so if the address didn't change since the last call
+		 * everything is fine and we can return early.
+		 * If not (which should only happen when some sort of bulk
+		 * conversion is implemented) we have to program the new
+		 * address. Note that this probably fails as the conversion that
+		 * was triggered above is like not complete yet and the two
+		 * operations have to be done in a single transfer.
+		 */
+		if (ddata->addr_prev == address)
+			return 0;
 	}
 
 	ret = i2c_smbus_write_byte(st->client,