msm: sensor: Validationg function pointers before using them

Since IOCTLS can come in any order, validating the actuator
function table and methods before accessing them.

CRs-Fixed: 1084177
Change-Id: Ic6fce52fdf4d1420c2b707ec9bc9cba045066a13
Signed-off-by: Sureshnaidu Laveti <lsuresh@codeaurora.org>