hwmon: (sht21) Fix documentation of struct sht21

Sparse reports:

drivers/hwmon/sht21.c:60: warning:
	No description found for parameter 'client'
drivers/hwmon/sht21.c:60:
	warning: Excess struct member 'hwmon_dev' description in 'sht21'
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/sht21.c

@@ -41,7 +41,7 @@
 
 /**
  * struct sht21 - SHT21 device specific data
- * @hwmon_dev: device registered with hwmon
+ * @client: I2C client device
  * @lock: mutex to protect measurement values
  * @last_update: time of last update (jiffies)
  * @temperature: cached temperature measurement value