pci: controller: msm: Prevent L0s before switching Gen speed

Link needs to be in L0 before Gen switching is performed. Programming
target link speed to HW does not take effect in L0s. After disabling
L0s perform a EP config read if the link is in L0s in order to bring
the link to L0 for Gen speed switch.

Change-Id: Iabe790b90b1da0421c2d59a7f2e053e5e8fe77c9
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>