hwmon: (sht15) Fix parameter documentation of sht15_crc8()

Sparse reports:

drivers/hwmon/sht15.c:188: warning:
		No description found for parameter 'len'
Signed-off-by: NGuenter Roeck <linux@roeck-us.net>

drivers/hwmon/sht15.c

@@ -179,6 +179,7 @@ struct sht15_data {
  * sht15_crc8() - compute crc8
  * @data:	sht15 specific data.
  * @value:	sht15 retrieved data.
+ * @len:	Length of retrieved data
  *
  * This implements section 2 of the CRC datasheet.
  */