arm: mvebu: a38x: serdes: Don't overwrite PCI device ID

PCI device ID is part of the PCIe controller SoC / revision. For Root
Complex mode (which is the default and the only mode supported currently
by U-Boot and Linux kernel), it is PCI device ID of PCIe Root Port device.

If there is some issue with this device ID, it should be set / updated by
PCIe controller driver (pci_mvebu.c), as this register resides in address
space of the controller. It shouldn't be done in SerDes initialization
code.

In the worst case (a specific board for example) it could be done via
U-Boot's weak function board_pex_config().

But it should not be overwritten globally for all A38x devices.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>