serial: sh-sci: prevent setup of uninitialized serial console

Commit 906b17dc introduced a condition
where the kernel will crash unless a earlyprintk parameter is specified.

Without this parameter, sci_console_init is called during early console
setup without any port being initialized, and the kernel crashes a
little bit later when uart_set_options attemps to invoke set_termios on a
port with an ops member equal to NULL.

This patch just checks in sci_console_init that the port is properly
initialized, and aborts the early console setup if it is not.

Signed-off-by: Alexandre Courbot <gnurou@gmail.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>