aq(4): arm PHY thermal shutdown only where a sensor exists

aq_fw2x_thermal_arm() reached for a copper PHY register that the fibre
parts do not implement, so arming failed on every init and printed a
warning for a capability the hardware cannot have. Return ENOTSUP when
the firmware does not advertise a temperature sensor, matching
aq_fw2x_get_temp(), and warn only for a genuine failure.

Signed-off-by: Nick Price <nprice@FreeBSD.org>
Accepted-by: adrian
Approved-by: adrian
(cherry picked from commit 3c7f1aa3b831431193106f8610b2142131d774f5)