fcoe: prep work to start consolidate the usage of fcoe_netdev

Currently, in the default kernel fcoe driver, it is needed to get to the underlying
private per fcoe transport's private structure, e.g., fcoe_interface in
fcoe.ko, and returns the associated netdev. The similar logic exists in other
fcoe drivers, e.g., bnx2fc, so we add a function pointer into the common
fcoe_port struct to allow individual fcoe transport implementaion (fcoe
and bnx2fc) to get the corresponding netdev associated with a give lport.

Then a inline fcoe_get_netdev() is added as part of libfcoe for all underlying
fcoe transport drivers to use regardless of its individual fcoe transport
driver, and also allows move more common code such as fcoe_link_speed_update or
fcoe_ctlr_get_lesb to be in libfcoe, rather than specific to fcoe.

This patch is a prep work that adds aforementioned fucntion pointer, and
followed by the actual code changes to make use of it.

Signed-off-by: Yi Zou <yi.zou@intel.com>
Cc: Bhanu Prakash Gollapudi <bprakash@broadcom.com>
Tested-by: Marcus Dennis <marcusx.e.dennis@intel.com>
Signed-off-by: Robert Love <robert.w.love@intel.com>