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: d503187b (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>