selftests: mlxsw: Use busywait helper in vxlan test

Vxlan test uses offload indication checks.

Use a busywait helper and wait until the offload indication is set or
fail if it reaches timeout.

Signed-off-by: Danielle Ratson <danieller@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>