ath10k: snoc: fix unabalanced regulator error handling

If a regulator fails to set its voltage, we end up with an unbalanced
call to regulator_disable(), because the error path starts with the
current regulator (which was never enabled).

Factor out the "on" function to perform (and unwind if failed) a single
regulator at a time, and then main loop (ath10k_snoc_vreg_on()) can just
worry about unwinding the regulators that were already enabled.

It also helps to factor out the "off" function, to avoid repeating some
code here.

Signed-off-by: Brian Norris <briannorris@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>