regulator: ltc3589: fix broken voltage transitions

VCCR is used as a trigger to start voltage transitions, so
we need to mark it volatile in order to make sure it gets
written to hardware every time we set a new voltage.

Fixes regulator voltage being stuck at the first voltage
set after driver load.

[lst: reworded commit message]
Signed-off-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org