libfcoe, fcoe: move fcoe_link_speed_update() to libfcoe and export it

With the previous patch, fcoe_link_speed_update() can be moved into libfcoe and
exported to used by fcoe, bnx2fc, and etc.

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>