USB: EHCI: Improve port index sanitizing

Now that Kees Cook has added a definition for HCS_N_PORTS_MAX in
commit 72dd1843 ("USB: EHCI: Add register array bounds to HCS
ports"), the code in ehci_hub_control() which sanitizes port index
values can be improved a little.

The idea behind this change is that it prevents a possible
out-of-bounds pointer computation, which the compiler might be able to
detect since the port_status[] array now has a fixed length rather
than a variable length.
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Link: https://lore.kernel.org/r/20211002190217.GA537967@rowland.harvard.eduSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/host/ehci-hub.c

@@ -745,12 +745,13 @@ int ehci_hub_control(
 	unsigned	selector;
 
 	/*
-	 * Avoid underflow while calculating (wIndex & 0xff) - 1.
-	 * The compiler might deduce that wIndex can never be 0 and then
-	 * optimize away the tests for !wIndex below.
+	 * Avoid out-of-bounds values while calculating the port index
+	 * from wIndex.  The compiler doesn't like pointers to invalid
+	 * addresses, even if they are never used.
 	 */
-	temp = wIndex & 0xff;
-	temp -= (temp > 0);
+	temp = (wIndex - 1) & 0xff;
+	if (temp >= HCS_N_PORTS_MAX)
+		temp = 0;
 	status_reg = &ehci->regs->port_status[temp];
 	hostpc_reg = &ehci->regs->hostpc[temp];