mhi: dev: net: remove using MHI skb priv structure

struct skb already has all the required information about device,
usage of private structure not required.

CRs-Fixed: 2258358
Change-Id: I479cb498834af8098a22fa82311b9fac18f16279
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>