msm: cpp: Checking for validity of regulator pointers

Not all targets have fs regulators, for such devices
crash is seen when firmware load fails. To fix this issue
validity check is made before disabling fs regulators.

Change-Id: Ib1b365bcdda724580e69d35dcf8dbeb25abc2b99
Signed-off-by: Shilpa Mamidi <shilpam@codeaurora.org>