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>