cnss2: Set initial target PCIe link speed to Gen2 for QCA6490

Since there may be link issues if QCA6490 device boots up with Gen3
link speed, always set initial target PCIe link speed to Gen2. Device
is able to change it (back) later at any time if needed.

Change-Id: I9d617d71926fe98e59e67aff772984878e0a2689
Signed-off-by: Yue Ma <yuem@codeaurora.org>