thunderbolt: nhi: Demote some non-conformant kernel-doc headers

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

 drivers/thunderbolt/nhi.c:53: warning: Function parameter or member 'ring' not described in 'ring_interrupt_active'
 drivers/thunderbolt/nhi.c:53: warning: Function parameter or member 'active' not described in 'ring_interrupt_active'
 drivers/thunderbolt/nhi.c:114: warning: Function parameter or member 'nhi' not described in 'nhi_disable_interrupts'
 drivers/thunderbolt/nhi.c:191: warning: Function parameter or member 'ring' not described in 'ring_write_descriptors'
 drivers/thunderbolt/nhi.c:225: warning: Function parameter or member 'work' not described in 'ring_work'

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>