Currently you can select HCDP independent of serial console.  This
doesn't seem very useful, and makes the setup_serial_hcdp() reference
unresolved if the serial driver is built as a module.

CONFIG_SERIAL_8250_CONSOLE is only selectable if the serial core is
built in (SERIAL_8250=y), so this patch makes sure we don't try to
call setup_serial_hcdp() unless it is actually built in to the kernel.

(I think we should also make HCDP selection dependent on
SERIAL_8250_CONSOLE=y in Kconfig; I'll send a separate patch
for that since it's not ia64-specific.)