media: s5k6aa: describe some function parameters

media: s5k6aa: describe some function parameters

as warned:
  drivers/media/i2c/s5k6aa.c:429: warning: No description found for parameter 's5k6aa'
  drivers/media/i2c/s5k6aa.c:679: warning: No description found for parameter 's5k6aa'
  drivers/media/i2c/s5k6aa.c:733: warning: No description found for parameter 's5k6aa'
  drivers/media/i2c/s5k6aa.c:733: warning: No description found for parameter 'preset'
  drivers/media/i2c/s5k6aa.c:787: warning: No description found for parameter 'sd'

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>