hwmon: twl4030-madc-hwmon: add "platform:" prefix for platform modalias

Since 43cc71ee (platform: prefix MODALIAS
with "platform:"), the platform modalias is prefixed with "platform:".

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Cc: J Keerthy <j-keerthy@ti.com>
Cc: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>