libfcoe, fcoe: consolidate the fcoe_ctlr_get_lesb/fcoe_get_lesb

Similarly they can be moved into libfcoe instead of being private to fcoe now.
Also add comments particularly on the term LESB to the corresponding function.

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>