arm: a37xx: pci: Fix pcie_advk_link_up()

Aardvark reports Disabled and Hot Reset LTSSM states as values >= 0x20.
Link is not up in these states, so fix pcie_advk_link_up() function.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>