thermal: Use IS_ENABLED instead of #ifdef

Use IS_ENABLED(CONFIG_THERMAL_EMULATION) to make the code more readable
and to get rid of the addtional #ifdef around the variable definitions
in thermal_zone_get_temp().

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Reviewed-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>