bonding: Add helper function to get the xmit slave in rr mode

Add helper function to get the xmit slave when bond is in round
robin mode. Change bond_xmit_slave_id to bond_get_slave_by_id, then
the logic for find the next slave for transmit could be used
both by the xmit flow and the .ndo to get the xmit slave.

Signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Jay Vosburgh <jay.vosburgh@canonical.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>