mlxsw: spectrum: Use NL_SET_ERR_MSG_MOD

Use NL_SET_ERR_MSG_MOD helper which adds the module name instead
of specifying the prefix each time.

Signed-off-by: Arkadi Sharshevsky <arkadis@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>