soc: qcom: bam_dmux: Correct payload length handing for dynamic mtu

Packets received under the dynamic mtu feature do not have a valid payload
size in the header, so the correct size is computed based on data from
SPS. This computation is incorrect, which results in the client
processing extra data in the scenario where their protocol requires the
packet to be dropped.

Correct the payload length handling for this scenario.

CRs-Fixed: 830835
Change-Id: I37e2d1bfe390657b9cdc2fcd530762d3daf97036
Signed-off-by: Jeffrey Hugo <jhugo@codeaurora.org>
Signed-off-by: Arun Kumar Neelakantam <aneela@codeaurora.org>