msm: camera: Set mctl client to NULL after destroy

After ion client is destroyed, mctl client is set
to NULL to avoid possibility of invalide client reuse.

Change-Id: Iacd752608affa2f17706d449200ad3fb9b650090
CRs-Fixed: 466520
Signed-off-by: Raju P.L.S.S.S.N <rplsssn@codeaurora.org>