twl4030-madc: Fix arbitrarily initialized function pointer

req is an automatic variable and thus we cannot rely on it being initialized to
zero (I am leaving the 0!= NULL discussion aside). Other functions test
if this pointer is NULL, in order to determine whether it is a valid address or
not.
Signed-off-by: Viktor Rosendahl <viktor.rosendahl@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>

drivers/i2c/chips/twl4030-madc.c

@@ -367,6 +367,7 @@ static int twl4030_madc_ioctl(struct inode *inode, struct file *filp,
 		req.channels = (1<<par.channel);
 		req.do_avg	= par.average;
 		req.method	= TWL4030_MADC_SW1;
+		req.func_cb	= NULL;
 
 		val = twl4030_madc_conversion(&req);
 		if (val <= 0) {