drivers: thermal: don't reset ops pointer in of-thermal sensor unregister

The thermal zone ops pointer is allocated and initialized during
building thermal zone at early boot. The framework expects this
variable is valid as long as that thermal zone exist. Later if
a sensor is registered to thermal zone and then unregistered the
same due to some reason, it resets this thermal zone ops variable.
It leads to unexpected behaviour whenever thermal framework tries
to use it.

Don't reset thermal zone ops variable during of-thermal sensor
unregister context.

Change-Id: I468cdfe08f4cde3fb1258ca335fe2ee8b457e014
Signed-off-by: Manaf Meethalavalappu Pallikunhi <manafm@codeaurora.org>