fcoe: handle CNAs with DCB firmware support

fcoe: handle CNAs with DCB firmware support

Some Combined Network Adapters(CNAs) implement DCB protocol
in firmware, it is recommended that do not run software-based
DCB or LLDP on CNAs that implement DCB, but we have to start
the lldpad service anyway(there might be other software DCB).

If the network interface provides hardware DCB/DCBX capabilities,
the field DCB_REQUIRED in "/etc/fcoe/cfg-xxx" is expected to
be set to "no".

We met an issue on "QLogic BCM57810" with DCB firmware support,
and found dracut still generated "fcoe=<mac>:dcb" which caused
kdump boot failure when using that fcoe dump target.

This patch parses /etc/fcoe/cfg-xxx to detect DCB_REQUIRED="no",
and force "nodcb" if it is the case.

Also improved some coding style in passing.

Signed-off-by: Xunlei Pang <xlpang@redhat.com>