PCI: dra7xx: Propagate platform_get_irq() errors in dra7xx_pcie_probe()

platform_get_irq() returns an error code, but the pci-dra7xx driver ignores
it and always returns -EINVAL. This is not correct and prevents
-EPROBE_DEFER from being propagated properly.

Print and propagate the return value of platform_get_irq() on failure.

This issue was detected with the help of Coccinelle.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Kishon Vijay Abraham I <kishon@ti.com>