msm: pcie: update mask and linkdown check in PCIe read/write API

The current check for linkdown does not suffice if the default
value of that register is 0xffffffff. PCIe bus driver needs to
also read the device/vendor ID of the client to confirm if it
is really a linkdown. Update the mask method to be compatible
with 64-bit architecture.

Change-Id: Ibe51dff50ff010260a750d73498333ee830fca2c
Signed-off-by: Tony Truong <truong@codeaurora.org>