ASoC: msm: qdsp6: check for invalid port index

Function which converts port ID to port index can return
error value if port ID is invalid. Caller should not
blindly use return value without checking

Change-Id: I02b8112b6714e3364758907e7078bcbb88d45b6b
Signed-off-by: Patrick Lai <plai@codeaurora.org>