mfd: tps65910: Correct power-off programming sequence

Correct power-off programming sequence in order to fix shutting down
devices which are using TPS65910 PMIC.

In accordance to the TPS65910 datasheet, the PMIC's state-machine
transitions into the OFF state only when DEV_OFF bit of DEVCTRL_REG is
set. The ON / SLEEP states also should be cleared, otherwise PMIC won't
get into a proper state on shutdown. Devices like Nexus 7 tablet and Ouya
game console are shutting down properly now.

Tested-by: Peter Geis <pgwipeout@gmail.com>
Tested-by: Zack Pearsall <zpearsall@yahoo.com>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>