thunderbolt: Use const qualifier for `ring_interrupt_index`

stable inclusion
from stable-v5.10.177
commit 2d6c2dee59a396b22fff6d708db4b167281e78c2
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I88YNP

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2d6c2dee59a396b22fff6d708db4b167281e78c2



--------------------------------

commit 1716efdb upstream.

`ring_interrupt_index` doesn't change the data for `ring` so mark it as
const. This is needed by the following patch that disables interrupt
auto clear for rings.

Cc: Sanju Mehta <Sanju.Mehta@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: sanglipeng <sanglipeng1@jd.com>
(cherry picked from commit aebf50e8)