[neighbour] Always use network device's own link-layer address

[neighbour] Always use network device's own link-layer address

The API for neighbour_tx() allows for an explicit source link-layer
address, but this will be ignored if the packet is deferred for
transmission after completion of neighbour discovery.  The network
device's own link-layer address will always be used when sending
neighbour discovery packets, and when sending any deferred packets
after discovery completes.

All callers pass in the network device's own link-layer address as the
source address anyway, and so this explicit source link-layer address
is never used for any meaningful purpose.

Simplify the neighbour_tx() API by removing the ability to pass in an
explicit source link-layer address.

Signed-off-by: Michael Brown <mcb30@ipxe.org>