msm: pcie: calculate ep's L1ss capability register offset

The address of L1ss capability registers varies depending on the
endpoint. This change calculates the endpoint's L1ss capability
register offset.

Change-Id: I6591624e2b1de1d5187912abb567776f9dcd22f7
Signed-off-by: Tony Truong <truong@codeaurora.org>