PCI: cadence: Correct probe behaviour when failing to get PHY

Test the correct value to see whether the PHY get failed.

Use devm_phy_get() instead of devm_phy_optional_get(), since it is
only called if phy name is given in devicetree and so should exist.
If failure when getting or linking PHY, put any PHYs which were
already got and unlink them.

Fixes: dfb80534 ("PCI: cadence: Add generic PHY support to host and EP drivers")
Reported-by: Colin King <colin.king@canonical.com>
Signed-off-by: Alan Douglas <adouglas@cadence.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>