CVE-2022-49392 serial: 8250_aspeed_vuart: Fix potential NULL dereference in aspeed_vuart_probe

In the Linux kernel, the following vulnerability has been resolved: serial: 8250aspeedvuart: Fix potential NULL dereference in aspeedvuartprobe platformgetresource may fail and return NULL, so we should better check it's return value to avoid a NULL pointer dereference...