ASoC: qdsp6v2: increase RTAC_MAX_ACTIVE_DEVICES to 6

To support up to 6 audio paths activated with different
devices, increase RTAC_MAX_ACTIVE_DEVICES from 4 to 6.

Change-Id: I16c1247b2b2bafdc646d412a75192494bf0e2b95
Signed-off-by: Derek Chen <chenche@codeaurora.org>