marvell/a3700: Prevent SError accessing PCIe link while it is down

When the link goes down (e.g. during a retrain), accessing the device
configuration space can trigger an ARM64 SError interrupt. Such
conditions cannot be predicted, so to avoid a crash the SError is
ignored.

Signed-off-by: Remi Pommarel <repk@triplefau.lt>
Change-Id: I2b1fd3296cc1c88b9ca1fe21c0924cb324eed58d