edgetpu: abrolhos: modify cooling device return value

To avoid thermal genl sending too many unnecessary events,
Modify the cooling device callback return value when the new
cooling action is the same as the old one.

Bug: 170682696
Test: run burn8 and only update cur_state when the new and old action
are different.

Change-Id: I163c9fa0d74e6744d3db10d95e4bfa654259890f
Signed-off-by: davidchao <davidchao@google.com>