staging: iio: tsl2x7x_core: fix format string warnings

Fixed the following warnings (reported by cppcheck):
[drivers/staging/iio/light/tsl2x7x_core.c:1150]: (warning) %d in format string (no. 1)
requires 'int' but the argument type is 'unsigned int'.
[drivers/staging/iio/light/tsl2x7x_core.c:1150]: (warning) %d in format string (no. 2)
requires 'int' but the argument type is 'unsigned int'.
[drivers/staging/iio/light/tsl2x7x_core.c:1150]: (warning) %d in format string (no. 3)
requires 'int' but the argument type is 'unsigned int'.
Signed-off-by: Asaf Vertz <asaf.vertz@tandemg.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/staging/iio/light/tsl2x7x_core.c

@@ -1147,7 +1147,7 @@ static ssize_t tsl2x7x_luxtable_show(struct device *dev,
 	int offset = 0;
 
 	while (i < (TSL2X7X_MAX_LUX_TABLE_SIZE * 3)) {
-		offset += snprintf(buf + offset, PAGE_SIZE, "%d,%d,%d,",
+		offset += snprintf(buf + offset, PAGE_SIZE, "%u,%u,%u,",
 			chip->tsl2x7x_device_lux[i].ratio,
 			chip->tsl2x7x_device_lux[i].ch0,
 			chip->tsl2x7x_device_lux[i].ch1);