PCI: mvebu: Dynamically detect if the PEX link is up to enable hot plug

Otherwise hotplugging the PEX doesn't work at all since the driver
detects the link state at probe time. Simply replacing the two tests
of haslink with a register read is enough to fix it.

Tested on kirkwood with repeated plug/unplug of the link partner.

Signed-off-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>