cnss: Don't set pci max link speed for pcie switch platform

The API - cnss_pci_set_max_link_speed sets max pcie link speed based
RC number, and affect other endpoints for PCIe switch platform. So,
don't set max link speed if dtsi configuration - "default_gen_speed"
is invalid.

Change-Id: I2b9263fb6c0b45dcb19cdc112fb04767648016f4
Signed-off-by: Wu Gao <wugao@codeaurora.org>