oem-ibm: Fix null buffer check in PCIeInfoHandler::write

The write operation is currently guarded by an inverted null
check, causing valid buffers to be ignored while allowing a
null pointer to be passed to pcieData.write().
Perform the write only when a valid buffer is provided.

Change-Id: Ib10401a2a239ee27cd819db6654928d4fbf99665
Signed-off-by: Meghana Vangapandu <meghanav@ami.com>