msm: pcie: use a local copy for PCIe event callback

Instead of directly modifying client's callback event, make a
local copy and give that to the client. This will prevent race
condition where the latest event is updated before the client
has had the chance to see the previous event.

Change-Id: Icb77471e71d48095eaa2610da26c22a161ff02e2
Signed-off-by: Tony Truong <truong@codeaurora.org>