[SERIAL] sunsab: Fix E250 console with RSC.

This fixes yet another sunsab problem, when console is set to anything
but the first port. The console framework calls sunsab_console_setup
for each port, and we end up setting up a console on a not yet
discovered port, which leads to an Oops. Instead, defer console setup
until the requested port is properly initialized. Tested on an E250
through an RSC console.

Reported by Daniel Smolik <marvin@mydatex.cz>

Signed-off-by: Marc Zyngier <maz@misterjones.org>
Signed-off-by: David S. Miller <davem@davemloft.net>