msm: camera: Use u64 to hold timestamp value

On 32-bit tv_sec is 4 bytes long and overflows in timestamp
computation.

Change-Id: I24387b5d8921e5f9cd0261beb7b179575f0b8a9d
Signed-off-by: Trishansh Bhardwaj <tbhardwa@codeaurora.org>