mlxsw: spectrum_switchdev: Add more extack messages

Add more extack messages that let the user know why VXLAN offload
failed.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Suggested-by: David Ahern <dsahern@gmail.com>
Cc: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>