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 <[e-mail]>
Signed-off-by: Lucas Stach <[e-mail]>
Signed-off-by: Mark Brown <[e-mail]>
Cc: [e-mail]

drivers/regulator/ltc3589.c

···
 	switch (reg) {
 	case LTC3589_IRQSTAT:
 	case LTC3589_PGSTAT:
+	case LTC3589_VCCR:
 		return true;
 	}
 	return false;