mfd: jz4740-adc: Use devm_kzalloc

Use devm_kzalloc and remove the error path free'ing and unload free'ing
as the devm resource functions free them.

Signed-off-by: Devendra Naga <devendra.aaru@gmail.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>