PCI: armada8k: Check the return value from clk_prepare_enable()

clk_prepare_enable() may fail, so check its return value and propagate it
in the case of error.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>