net: dsa: dump CPU port regs through master

Merge the CPU port registers dump into the master interface registers
dump through ethtool, by nesting the ethtool_drvinfo and ethtool_regs
structures of the CPU port into the dump.

drvinfo->regdump_len will contain the full data length, while regs->len
will contain only the master interface registers dump length.

This allows for example to dump the CPU port registers on a ZII Dev
C board like this:

    # ethtool -d eth1
    0x004:                                              0x00000000
    0x008:                                              0x0a8000aa
    0x010:                                              0x01000000
    0x014:                                              0x00000000
    0x024:                                              0xf0000102
    0x040:                                              0x6d82c800
    0x044:                                              0x00000020
    0x064:                                              0x40000000
    0x084: RCR (Receive Control Register)               0x47c00104
        MAX_FL (Maximum frame length)                   1984
        FCE (Flow control enable)                       0
        BC_REJ (Broadcast frame reject)                 0
        PROM (Promiscuous mode)                         0
        DRT (Disable receive on transmit)               0
        LOOP (Internal loopback)                        0
    0x0c4: TCR (Transmit Control Register)              0x00000004
        RFC_PAUSE (Receive frame control pause)         0
        TFC_PAUSE (Transmit frame control pause)        0
        FDEN (Full duplex enable)                       1
        HBC (Heartbeat control)                         0
        GTS (Graceful transmit stop)                    0
    0x0e4:                                              0x76735d6d
    0x0e8:                                              0x7e9e8808
    0x0ec:                                              0x00010000
    .
    .
    .
    88E6352  Switch Port Registers
    ------------------------------
    00: Port Status                            0x4d04
          Pause Enabled                        0
          My Pause                             1
          802.3 PHY Detected                   0
          Link Status                          Up
          Duplex                               Full
          Speed                                100 or 200 Mbps
          EEE Enabled                          0
          Transmitter Paused                   0
          Flow Control                         0
          Config Mode                          0x4
    01: Physical Control                       0x003d
          RGMII Receive Timing Control         Default
          RGMII Transmit Timing Control        Default
          200 BASE Mode                        100
          Flow Control's Forced value          0
          Force Flow Control                   0
          Link's Forced value                  Up
          Force Link                           1
          Duplex's Forced value                Full
          Force Duplex                         1
          Force Speed                          100 or 200 Mbps
    .
    .
    .

Signed-off-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>