bcm4908: enable & setup packet steering

bcm4908: enable & setup packet steering

Without packet steering NAT masquarade speed on BCM4908 /jumps/ between
two speeds:
1. 826 Mb/s (±3 Mb/s)
2. 909 Mb/s (±8 Mb/s)
and it never reaches ~940 Mb/s.

Proper packet steering can improve it. Below are testing results for
running iperf TCP traffic from LAN to WAN. They were used to pick up
golden values.

┌──────────┬──────────┬───────────┐
│   eth0   │  br-lan  │   speed   │
│ rps_cpus │ rps_cpus │   [Mbps]  │
├──────────┼──────────┼───────────┤
│        0 │        0 │ 743 / 804 │
│        0 │        1 │ 738 / 821 │
│        0 │        2 │     ✓ 940 │
│        0 │        4 │     ✓ 938 │
│        0 │        8 │     ✓ 941 │
├──────────┼──────────┼───────────┤
│        1 │        0 │       829 │
│        1 │        1 │       829 │
│        1 │        2 │     ✓ 942 │
│        1 │        4 │     ✓ 941 │
│        1 │        8 │     ✓ 941 │
├──────────┼──────────┼───────────┤
│        2 │        0 │     ✓ 942 │
│        2 │        1 │       926 │
│        2 │        2 │     ✓ 942 │
│        2 │        4 │     ✓ 942 │
│        2 │        8 │     ✓ 941 │
├──────────┼──────────┼───────────┤
│        4 │        0 │     ✓ 941 │
│        4 │        1 │       925 │
│        4 │        2 │     ✓ 941 │
│        4 │        4 │     ✓ 941 │
│        4 │        8 │     ✓ 941 │
├──────────┼──────────┼───────────┤
│        8 │        0 │     ✓ 942 │
│        8 │        1 │       925 │
│        8 │        2 │     ✓ 941 │
│        8 │        4 │     ✓ 942 │
│        8 │        8 │     ✓ 942 │
└──────────┴──────────┴───────────┘

Ref: fcbd39689ebfe ("bcm53xx: enable & setup packet steering")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>