serial: samsung: Fix unintended usage of uart port 0 as console

In s3c24xx_serial_console_setup function, if the uart port that is
being setup as a console has not been initialized, an error can be
returned instead of using uart port 0 as the default console port.
The uart port that was intended to be used as a console could be
initialized at a later point during boot and then registered as a
console. This will avoid using uart port 0 as a unintended console
port.

Signed-off-by: Thomas Abraham <thomas.ab@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>