mfd: Check for twl4030-madc NULL pointer

If the twl4030-madc device wasn't registered, and another device, such
as twl4030-madc-hwmon, calls twl4030_madc_conversion() a NULL pointer is
dereferenced.
Signed-off-by: Kyle Manna <kyle@kylemanna.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

drivers/mfd/twl4030-madc.c

@@ -510,8 +510,9 @@ int twl4030_madc_conversion(struct twl4030_madc_request *req)
 	u8 ch_msb, ch_lsb;
 	int ret;
 
-	if (!req)
+	if (!req || !twl4030_madc)
 		return -EINVAL;
+
 	mutex_lock(&twl4030_madc->lock);
 	if (req->method < TWL4030_MADC_RT || req->method > TWL4030_MADC_SW2) {
 		ret = -EINVAL;