PCI: mvebu: Check valid base address before port setup

This driver does not fail to probe when it cannot obtain
a port base address. Therefore, add a check for NULL base address
before setting up the port, which prevents a kernel panic in such
cases.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Tested-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>