usb: host: xhci: mvebu: Iterate over array indexes instead of using pointer math

Walking the dram->cs array was seen as accesses beyond the first array item by the compiler. Instead, use the array index directly. This allows for run-time bounds checking under CONFIG_UBSAN_BOUNDS as well. Seen with GCC 13 with -fstrict-flex-arrays:

In function 'xhci_mvebu_mbus_config',
inlined from 'xhci_mvebu_mbus_init_quirk' at ../drivers/usb/host/xhci-mvebu.c:66:2:
../drivers/usb/host/xhci-mvebu.c:37:28: warning: array subscript 0 is outside array bounds of 'const struct mbus_dram_window[0]' [-Warray-bounds=]
37 | writel(((cs->size - 1) & 0xffff0000) | (cs->mbus_attr << 8) |

      |                          ~~^~~~~~

---
 drivers/usb/host/xhci-mvebu.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 60651a50770f..87f1597a0e5a 100644
@@ -32,7 +32,7 @@ static void xhci_mvebu_mbus_config(void __iomem *base,
 
 	/* Program each DRAM CS in a seperate window */
 	for (win = 0; win < dram->num_cs; win++) {
-		const struct mbus_dram_window *cs = dram->cs + win;
+		const struct mbus_dram_window *cs = &dram->cs[win];
 
 		writel(((cs->size - 1) & 0xffff0000) | (cs->mbus_attr << 8) |
 		       (dram->mbus_dram_target_id << 4) | 1,

2.34.1