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>