lldp packet transmission on a bonded interface

LLDP packets are dropped at interface output node if each slave's link
is configured as the LLDP interface. The admin state is configured and
managed by the bonded interface, so slave link's state is down by default.
The checking for the admin state UP should be ignored for the slave link.

Change-Id: I06ca250f42fcb8cc50e0ea3a3817a2c5b56865df
Signed-off-by: Steve Shin <jonshin@cisco.com>