msm: pcie: replace all memory barriers with readbacks

Memory barrier cannot fully guarantee the write has completed
before issuing the next one. Replace all memory barriers with
readbacks to ensure the writes have completed.

Change-Id: I5d9f3b5df4786d7d4bca096ce04aff83ddc2d381
Signed-off-by: Tony Truong <truong@codeaurora.org>