PCI: armada8k: Fix platform_get_irq() error handling

platform_get_irq() returns a negative number on failure, so adjust the
logic to detect such condition and propagate the real error value on
failure.

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