msm: board-8960: Fix overflow in memory calc.

An overflow occurs in locate_unstable_memory() when physical
memory address space ends at 0xFFFFFFFF. Change code to
handle overflow by ensuring that variables are limited to
0xFFFFFFFF and by rearranging additions and subtractions.

Change-Id: I5839499d8274214ca08b9c11b57d44861ec894a3
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>