drivers/net/bonding: fix sparse warning: symbol shadows an earlier one

Impact: Rename function scope variable.

Fix this sparse warning:
  drivers/net/bonding/bond_main.c:4704:13: warning: symbol 'mode' shadows an earlier one
  drivers/net/bonding/bond_main.c:95:13: originally declared here

Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Signed-off-by: David S. Miller <davem@davemloft.net>