media: radio-si476x: fix kernel-doc markups

get rid of the following warnings:
  drivers/media/radio/radio-si476x.c:317: warning: No description found for parameter 'v4l2dev'
  drivers/media/radio/radio-si476x.c:317: warning: No description found for parameter 'ctrl_handler'
  drivers/media/radio/radio-si476x.c:317: warning: No description found for parameter 'debugfs'
  drivers/media/radio/radio-si476x.c:317: warning: No description found for parameter 'audmode'
  drivers/media/radio/radio-si476x.c:317: warning: Excess struct member 'kref' description in 'si476x_radio'
  drivers/media/radio/radio-si476x.c:317: warning: Excess struct member 'core_lock' description in 'si476x_radio'

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