USB: serial: io_ti: fix array underflow in completion handler

As reported by Dan Carpenter, a malicious USB device could set port_number to -3 and we would underflow the port array in the interrupt completion handler.

As these devices only have one or two ports, fix this by making sure we only consider the seventh bit when determining the port number (and ignore bits 0xb0 which are typically set to 0x30).

---
 drivers/usb/serial/io_ti.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index e53c68261017..9bbcee37524e 100644
@@ -173,7 +173,7 @@ struct ump_interrupt {
 }  __attribute__((packed));
 
 
-#define TIUMP_GET_PORT_FROM_CODE(c)	(((c) >> 4) - 3)
+#define TIUMP_GET_PORT_FROM_CODE(c)	(((c) >> 6) & 0x01)
 #define TIUMP_GET_FUNC_FROM_CODE(c)	((c) & 0x0f)
 #define TIUMP_INTERRUPT_CODE_LSR	0x03
 #define TIUMP_INTERRUPT_CODE_MSR	0x04