thunderbolt: Use const qualifier for `ring_interrupt_index`

`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>

drivers/thunderbolt/nhi.c

@@ -46,7 +46,7 @@
 #define QUIRK_AUTO_CLEAR_INT	BIT(0)
 #define QUIRK_E2E		BIT(1)
 
-static int ring_interrupt_index(struct tb_ring *ring)
+static int ring_interrupt_index(const struct tb_ring *ring)
 {
 	int bit = ring->hop;
 	if (!ring->is_tx)