mlxsw: spectrum: Move flow offload binding into spectrum_flow.c

Move the code taking case of setup of flow offload into spectrum_flow.c
Do small renaming of callbacks on the way.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>