Silicon/Broadcom/Bcm27xx: Move linkup check into the cfg accessor

The existing code fails to create/finish configuring the
pcie subsystem if it fails to get a linkup. This is reasonable
on the RPi4 because it generally won't happen, and the OS
could not see the root port. Now that the OS can see the
root port, its a bit odd if it only shows up when
something is plugged into the first slot. Lets move the
link up check into the config accessor where it will be used
to restrict sending CFG TLP's out the port when nothing is
plugged in. Thus avoiding a SERROR during probe.

Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
Reviewed-by: Andrei Warkentin <awarkentin@vmware.com>
Tested-by: Jared McNeill <jmcneill@invisible.ca>