ASoC: msm: qdsp6v2: Add upper bounds check on index

Add upper bounds check for return value of
q6audio_get_port_index().

CRs-Fixed: 575227
Change-Id: I263fc6d759beffd2c2fecc7794c279939ab25401
Signed-off-by: Kenneth Westfield <kwestfie@codeaurora.org>