drivers: adc_ad4130|adc_7124: Add ret initialization in *_perform_read()

Add initialization of `ret` to avoid reports of uninitialized variable
being returned. This variable normally gets initialized while the loop
iterates over channels, but potentially it could be left uninitialized
if the loop finished in its first iteration because of no channels to
be processed.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>