msm: camera: sync: Fix kbuild config checks for synx driver

External configs should be checked by using IS_REACHABLE/IS_ENABLED
macro's to check if the config option is set for the build target.

CRs-Fixed: 2584631
Change-Id: I07a0d95391a83a2264f826e2935d67686d295317
Signed-off-by: Karthik Jayakumar <kjayakum@codeaurora.org>