If the new governor fails to start, switch back to old governor so that the
devfreq state is not left in some weird limbo.

[Myungjoo: assume fatal on revert failure and set df->governor to NULL]
Signed-off-by: NSibi Sankar <sibis@codeaurora.org>
Signed-off-by: NSaravana Kannan <skannan@codeaurora.org>
Reviewed-by: NChanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: NMyungJoo Ham <myungjoo.ham@samsung.com>