bonding/alb: Add helper functions to get the xmit slave

Add two helper functions to get the xmit slave of bond in alb or tlb
mode. Extract the logic of find the xmit slave from the xmit flow
to function. Xmit flow will xmit through this slave and in the
following patches the new .ndo will call to the helper function
to return 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>