dcbnl: Shorten all command handling functions

Allocating and sending the skb in dcb_doit() allows for much
shorter and cleaner command handling functions.

The huge switch statement is replaced with an array based definition
of the handling function and reply message type.
Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>