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>