IB/core: Add link layer property to ports

This patch allows ports to have different link layers:
IB_LINK_LAYER_INFINIBAND or IB_LINK_LAYER_ETHERNET.  This is required
for adding IBoE (InfiniBand-over-Ethernet, aka RoCE) support.  For
devices that do not provide an implementation for querying the link
layer property of a port, we return a default value based on the
transport: RMA_TRANSPORT_IB nodes will return IB_LINK_LAYER_INFINIBAND
and RDMA_TRANSPORT_IWARP nodes will return IB_LINK_LAYER_ETHERNET.

Signed-off-by: Eli Cohen <eli@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>