msm: cam_smmu: Fix page fault handler to return proper error code

A recent change in the ARM SMMU driver expects either a 0 or an
-EBUSY from any of the registered page fault handlers. Otherwise
it will assume the page fault unhandled and trigger a fatal crash.
So unless the various context bank page fault handlers are updated
to return either a 0 or an -EBUSY, each page fault will trigger a
fatal crash. This will make debugging the underlying problem
causing the page fault in the first place, more difficult.

So fix the page fault handler to return proper error code so that
ARM SMMU driver does not consider the page fault unhandled and the
device does not crash.

Change-Id: I1f1cab324d6ee89400cd7006758a906244368472
Signed-off-by: Seemanta Dutta <seemanta@codeaurora.org>