msm: ipa: add support to specify Ethernet header offset attribute

Clients installing headers can now specify the offset of the
Ethernet-II (or 802.3) header within the overall header if
applicable. This enables configuration logic to not have to
hard-code such offset knowledge.

Change-Id: Ib3e835e6c28358b2a9be147333c2c89bd8f495ed
Signed-off-by: Ravi Gummadidala <rgummadi@codeaurora.org>