msm: thermal-dev: Protect IOCTL from race condition

There is a possibility that the thermal ioctl interface can be accessed
simultaneously in a multi-threaded environment. In those cases the
calls to fetch the frequency and voltage table can result in an
undefined behavior due to race condition.

Use mutex to protect the IOCTL interface from multi-thread access race
condition.

Change-Id: I325695f38753a4d4bc732987cf514e8616273aca
Signed-off-by: Ram Chandrasekar <rkumbako@codeaurora.org>