coreaudio: add missing aout_FormatPrepare()

It was missing after the format was being set to FL32, causing i_bytes_per_frame mismatch when the input format size was different than 4.

It is already done in MapOutputLayout() since this function change the number of channels but it was not done for the MapInputLayout() case.

Regression from 87bbf8f2

Fixes #28117 (closed)