msm: pcie: Use local variable for manipulation

There is a chance that wr_offset can be modified to
an arbitrary value as it is a global variable. Have
a local copy of this value and use it for further
manipulation.

Change-Id: If3b76a0dd95e81bd057d89626818c72405f91d65
Signed-off-by: Rama Krishna Phani A <rphani@codeaurora.org>