lib_i2s receive specifies const samples, but sometimes we want to do in-place DSP

[ed-xmos]

https://github.com/xmos/fwk_io/blob/develop/modules/i2s/api/i2s.h#L116

To keep the compiler happy, we are making a copy. Typically a few bytes and a few cycles but worth changing in a future update.

The use case is where we do far-end DSP in a voice product which requires modifying of the received samples before sending to the application.