extcon: gpio: Use power efficient workqueue for debounce

The debounce timeout is generally quite long and the work not performance
critical so allow the scheduler to run the work anywhere rather than in
the normal per-CPU workqueue.

Signed-off-by: Mark Brown <broonie@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Myungjoo Ham <myungjoo.ham@samsung.com>
(cherry picked from commit d0db2e7ae788d84ff6d0a1cd4dc935282db29073)