Refactor audio_processing/nsx: Removed usage of macro WEBRTC_SPL_MEMCPY_W16

The macro assumes int16_t pointers, but there is no check for it.

BUG=3348,3353
TESTED=locally on Linux and trybots
R=kwiberg@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/48959004

Cr-Commit-Position: refs/heads/master@{#8987}
3 files changed