soc: qcom: bam_dmux: Use skb tail helper functions

Accessing or modifying the tail member of a skb should be done through the
helper functions as the tail member can be represented as one of multiple
forms internally in the skb.

Use the skb tail helper functions where appropiate to access or modify the
tail member.

Change-Id: Ib2c8788259a5a2d404781e9a1be537105d4031d4
Signed-off-by: Jeffrey Hugo <jhugo@codeaurora.org>