thermal: testing: Simplify tt_get_tt_zone()

Notice that tt_get_tt_zone() need not use the ret variable in
the tt_thermal_zones list walk and make it return right after
incrementing the reference counter of the matching entry.

No functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://patch.msgid.link/6120304.lOV4Wx5bFT@rjwysocki.net