net: offload: Move offload_ip field from net_l2 to net_if

This is part of a patch series to decouple TCP/IP offload, which
currently occurs at the transport layer (L4), from the data link
layer (L2).

This provides a more direct call stack from the net_context APIs
to the offload driver functions, versus going through the current
offload L2 (dummy) driver.

Change-Id: Ia5a677d87f7b62f54b2702ce8ecfaf026f6f7c26
Signed-off-by: Gil Pitney <gil.pitney@linaro.org>