serial: earlycon: Only try fdt when specify 'earlycon' exactly

When moving earlycon early_param handling to serial, the devicetree
earlycons enable condition changed slightly.

We used to only do that for 'console'/'earlycon', but now would also
for 'console='/'earlycon='.

Fix it by using the same condition like before.

Fixes: d503187b6cc4 (of/serial: move earlycon early_param handling to serial)
Signed-off-by: Jeffy Chen <jeffy.chen@rock-chips.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/earlycon.c

@@ -220,7 +220,7 @@ static int __init param_setup_earlycon(char *buf)
 		if (IS_ENABLED(CONFIG_ACPI_SPCR_TABLE)) {
 			earlycon_init_is_deferred = true;
 			return 0;
-		} else {
+		} else if (!buf) {
 			return early_init_dt_scan_chosen_stdout();
 		}
 	}