Testing uncovered that even when the taprio gate is closed, some packets
still transmit.

According to i225/6 hardware errata [1], traffic might overflow the
planned QBV window. This happens because MAC maintains an internal buffer,
primarily for supporting half duplex retries. Therefore, even when the
gate closes, residual MAC data in the buffer may still transmit.

To mitigate this for i226, reduce the MAC's internal buffer from 192 bytes
to the recommended 88 bytes by modifying the RETX_CTL register value.

This follows guidelines from:
[1] Ethernet Controller I225/I22 Spec Update Rev 2.1 Errata Item 9:
    TSN: Packet Transmission Might Cross Qbv Window
[2] I225/6 SW User Manual Rev 1.2.4: Section 8.11.5 Retry Buffer Control

Note that the RETX_CTL register can't be used in TSN mode because half
duplex feature cannot coexist with TSN.

Test Steps:
1.  Send taprio cmd to board A:
    tc qdisc replace dev enp1s0 parent root handle 100 taprio \
    num_tc 4 \
    map 3 2 1 0 3 3 3 3 3 3 3 3 3 3 3 3 \
    queues 1@0 1@1 1@2 1@3 \
    base-time 0 \
    sched-entry S 0x07 500000 \
    sched-entry S 0x0f 500000 \
    flags 0x2 \
    txtime-delay 0

    Note that for TC3, gate should open for 500us and close for another
    500us.

3.  Take tcpdump log on Board B.

4.  Send udp packets via UDP tai app from Board A to Board B.

5.  Analyze tcpdump log via wireshark log on Board B. Ensure that the
    total time from the first to the last packet received during one cycle
    for TC3 does not exceed 500us.

Fixes: 43546211 ("igc: Add new device ID's")
Signed-off-by: Faizal Rahim <faizal.abdul.rahim@linux.intel.com>
Acked-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Tested-by: Mor Bar-Gabay <morx.bar.gabay@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>