mlxsw: spectrum_acl: Put vchunk migrate start/end code into separate functions

In preparations of interrupt/continue of rehash work, put the code that
is done at the beginning/end of vchunk migrate function into separate
functions.

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>