thermal: exynos: Correct sanity check at exynos_report_trigger() function

Up till now, by mistake, wrong variable was tested against being NULL.
Since exynos_report_trigger() is always called with valid p pointer,
it is only necessary to check if a valid thermal zone device is passed.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>