x86: acpi: Not every platform has serial console a first device

x86: acpi: Not every platform has serial console a first device

We may not do an assumption that current console device is always a first
of UCLASS_SERIAL one.

For example, on properly described Intel Edison board the console UART
is a third one.

Use current serial device as described in global data.

Fixes: a61cbad78e67 ("dm: serial: Adjust serial_getinfo() to use proper API")
Cc: Simon Glass <[email protected]>
Signed-off-by: Andy Shevchenko <[email protected]>
Reviewed-by: Bin Meng <[email protected]>