serial: 8250_aspeed_vuart: Fix potential NULL dereference in aspeed_vuart_probe

platform_get_resource() may fail and return NULL, so we should
better check it's return value to avoid a NULL pointer dereference.

Fixes: 54da3e38

 ("serial: 8250_aspeed_vuart: use UPF_IOREMAP to set up register mapping")
Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Link: https://lore.kernel.org/r/20220404143842.16960-1-linmq006@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>