PCI: keystone: Do not initiate link training multiple times

commit 886bc5ce

 ("PCI: designware: Add generic
dw_pcie_wait_for_link()") while adding a generic dw_pcie_wait_for_link()
performed a special handling (initiate link training multiple times) for
keystone which is not required. This also resulted in unncessarily waiting
for more time to establish the link even when no PCI device is connected.

Remove it and make it look similar to other dwc based PCIe drivers.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>