msm: pcie: remove max link speed property

PCIe bus driver no longer needs to differentiate between the
type of cores since the settings are compatbile with all.
Therefore, remove the max link speed property.

Change-Id: I5935c2c0794eb62fe6a84ed5551c664b2cd89d43
Signed-off-by: Tony Truong <truong@codeaurora.org>