serial: sh-sci: Limit early console to one device.

Presently the early console setup will be invoked for every device handed
off to he earlyprintk command line argument. In practice we can only
handle one, so this adds a sanity check to prevent clobbering the
existing active console.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>

diff --cc drivers/serial/sh-sci.c
Simple merge