thermal: bxt: remove redundant variable trip

Variable trip is assigned but never read, hence it is redundant
and can be removed. Cleans up clang warning:

drivers/thermal/intel_bxt_pmic_thermal.c:204:4: warning: Value stored
to 'trip' is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>