dm: serial: Correct logic in serial_find_console_or_panic()

In a final attempt to find a console UART this function uses the first
first available serial device. However the check for a valid device is
inverted.

This code is only executed when there is in fact no serial UART, but at
present it can fail to reach the panic_str() call in this case, and start
trying to use a non-existent UART.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>