msm: camera: gen buf mng: Fix compat error handling

In compat implementation returned if internal
handler fails. This causes multiple issues
in userspace where call succeed but buffer
index is not vaild.

Change-Id: I970134ff65c5b0f0aa6b00b62439ceb20462aa5f
Signed-off-by: Gjorgji Rosikopulos <grosik@codeaurora.org>