i40e: fix unrecognized FCOE EOF case

Because i40e_fcoe_ctxt_eof should never be called without
i40e_fcoe_eof_is_supported being called first, the EOF in fcoe_ctxt_eof
should always be valid and therefore we do not need to print an error
if it is not valid.

However, a WARN ON to easily catch any calls to i40e_fcoe_ctxt_eof that
aren't preceded with a call to i40e_fcoe_eof_is_supported is helpful.

Change-ID: I3b536b1981ec0bce80576a74440b7dea3908bdb9
Signed-off-by: Vasu Dev <vasu.dev@intel.com>
Tested-by: Jim Young <james.m.young@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>