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
diff --git a/drivers/regulator/ltc3589.c b/drivers/regulator/ltc3589.c
index c756955..0ce8e4e 100644
--- a/drivers/regulator/ltc3589.c
+++ b/drivers/regulator/ltc3589.c
@@ -372,6 +372,7 @@
 	switch (reg) {
 	case LTC3589_IRQSTAT:
 	case LTC3589_PGSTAT:
+	case LTC3589_VCCR:
 		return true;
 	}
 	return false;