Rename function extract_ac_13_12() to extract_ac_15_14().

Signed-off-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Reviewed-by: Aleksandar Rikalo <arikalo@wavecomp.com>
Message-Id: <1551185735-17154-3-git-send-email-aleksandar.markovic@rt-rk.com>