PCI: rcar: Check platform_get_irq() return code

The current code does not check the return from platform_get_irq() so add
an error check and return if this call does fail.

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Simon Horman <horms+renesas@verge.net.au>