thermal: core: Remove excess empty line from a comment

The first and the third lines of the kerneldoc comment for:

thermal_zone_device_set_polling()

belong to the same sentences, so join them together.

Signed-off-by: Flavio Suligoi <f.suligoi@asem.it>
[ rjw: Subject and changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>