iio: chemical: sgp30: Add description for sgp_read_cmd()'s 'duration_us'

Fixes the following W=1 kernel build warning(s):

 drivers/iio/chemical/sgp30.c:236: warning: Function parameter or member 'duration_us' not described in 'sgp_read_cmd'

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Cc: Andreas Brauchli <a.brauchli@elementarea.net>
Cc: Pascal Sachs <pascal.sachs@sensirion.com>
Link: https://lore.kernel.org/r/20200716135928.1456727-6-lee.jones@linaro.org


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>