hal: spkr_prot: Update the thermal zone path to physical path

When symbolic link is used instead of physical path to access
the thermal zones, hal fails to access the zones sometimes
due to insufficient access rights to sysfs.
Update the thermal zone path to sys/devices/virtual/thermal from
sys/class/thermal which is a symbolic link in order to access
the zones without fail.

Change-Id: I0850444f662ffe7dc7cc4076adeba8dc36fbc687