PCI: keystone: Fix link training retries initiation

ks_pcie_stop_link() function does not clear LTSSM_EN_VAL bit so
link training was not triggered more than once after startup.
In configurations where link can be unstable during early boot,
for example, under low temperature, it will never be established.

Fixes: 0c4ffcfe

 ("PCI: keystone: Add TI Keystone PCIe driver")
Signed-off-by: Yurii Monakov <monakov.y@gmail.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Andrew Murray <andrew.murray@arm.com>
Cc: stable@vger.kernel.org