cnss2: Avoid trying DRV suspend if PCIe link speed is Gen 3

Commit baa52351 ("cnss2: change for fixed link speed") has a side
effect that driver may try to enter DRV suspend with Gen 3 link speed
which is currently not supported. This change will fix the original
issue and side effect properly.

Change-Id: Ia7dbee86554be7ddcec0fc1a91ad6ffbc7d6b2e5
Signed-off-by: Yue Ma <yuem@codeaurora.org>