cnss2: Set pcie link speed as default_gen_speed in dts

For qca6490, read dts first to get default gen speed, if valid
setting existing, then use it, else use gen2.

Change-Id: I6cb507746d677da114408e78076c3cc21073a829
CRs-Fixed: 3003876
Signed-off-by: Jianmin Zhu <jianminz@codeaurora.org>