msm: rtb: Disable RTB on panic

By the time the panic notifiers have run, all the useful
information for debugging purposes has already been collected.
Stop logging during panic so useful information doesn't get
flooded out.

Change-Id: I7e23db3d6d3dfaf53cad86b2949a550d7cf218d1
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>