bonding: use pre-defined macro in bond_mode_name instead of magic number 0

We have BOND_MODE_ROUNDROBIN pre-defined as 0, and it's the lowest
mode number.
Use it to check the arg lower bound instead of magic number 0 in
bond_mode_name.

Signed-off-by: Wang Sheng-Hui <shhuiw@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>