[BRIDGE]: br_dump_ifinfo index fix

Fix for inability of br_dump_ifinfo to handle non-zero start index:
loop index never increases when entered with non-zero start.
Spotted by Kirill Korotaev.

Signed-off-by: Andrey Savochkin <saw@swsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>