msm: pcie: only look for EP CAP reg for certain testcases

Not all the testcases for debugfs needs the calculated offset
of an endpoint's capability register. Therefore, only calculate
the offset of an endpoint's capanility register if that testcase
needs it.

Change-Id: Iffddcea682d8c9344f51a04b57f60ba906b01dc6
Signed-off-by: Tony Truong <truong@codeaurora.org>