power: supply: ab8500: Defer thermal zone probe

The call thermal_zone_get_zone_by_name() used to return the
thermal zone right away, but recent refactorings in the
thermal core has changed this so the thermal zone used by
the battery is probed later, and the call returns -ENODEV.

This was always quite fragile. If we get -ENODEV, then
return a -EPROBE_DEFER and try again later.

Cc: phone-devel@vger.kernel.org
Fixes: 2b0e7ac0 ("power: supply: ab8500: Integrate thermal zone")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>