mfd: Cache wm8994 chip revision

There's no need to mark the chip revision registers as volatile, it won't
change at runtime so we can cache it from the device at startup.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

drivers/mfd/wm8994-regmap.c

@@ -1122,7 +1122,6 @@ static bool wm8994_volatile_register(struct device *dev, unsigned int reg)
 	case WM8994_RATE_STATUS:
 	case WM8958_MIC_DETECT_3:
 	case WM8994_DC_SERVO_4E:
-	case WM8994_CHIP_REVISION:
 	case WM8994_INTERRUPT_STATUS_1:
 	case WM8994_INTERRUPT_STATUS_2:
 		return true;