thunderbolt: ctl: Demote non-conformant kernel-doc headers

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

 drivers/thunderbolt/ctl.c:38: warning: expecting prototype for struct tb_cfg. Prototype was for struct tb_ctl instead
 drivers/thunderbolt/ctl.c:350: warning: Function parameter or member 'ctl' not described in 'tb_ctl_tx'
 drivers/thunderbolt/ctl.c:350: warning: Function parameter or member 'data' not described in 'tb_ctl_tx'
 drivers/thunderbolt/ctl.c:350: warning: Function parameter or member 'len' not described in 'tb_ctl_tx'
 drivers/thunderbolt/ctl.c:350: warning: Function parameter or member 'type' not described in 'tb_ctl_tx'
 drivers/thunderbolt/ctl.c:350: warning: expecting prototype for tb_cfg_tx(). Prototype was for tb_ctl_tx() instead
 drivers/thunderbolt/ctl.c:383: warning: Function parameter or member 'ctl' not described in 'tb_ctl_handle_event'
 drivers/thunderbolt/ctl.c:383: warning: Function parameter or member 'type' not described in 'tb_ctl_handle_event'
 drivers/thunderbolt/ctl.c:383: warning: Function parameter or member 'pkg' not described in 'tb_ctl_handle_event'
 drivers/thunderbolt/ctl.c:383: warning: Function parameter or member 'size' not described in 'tb_ctl_handle_event'

Cc: Andreas Noever <andreas.noever@gmail.com>
Cc: Michael Jamet <michael.jamet@intel.com>
Cc: Yehezkel Bernat <YehezkelShB@gmail.com>
Cc: linux-usb@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
[ mw: Demote only static functions ]
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>