net: mscc: ocelot: fix QoS class for injected packets with "ocelot-8021q"

There are 2 distinct code paths (listed below) in the source code which
set up an injection header for Ocelot(-like) switches. Code path (2)
lacks the QoS class and source port being set correctly. Especially the
improper QoS classification is a problem for the "ocelot-8021q"
alternative DSA tagging protocol, because we support tc-taprio and each
packet needs to be scheduled precisely through its time slot. This
includes PTP, which is normally assigned to a traffic class other than
0, but would be sent through TC 0 nonetheless.

The code paths are:

(1) ocelot_xmit_common() from net/dsa/tag_ocelot.c - called only by the
    standard "ocelot" DSA tagging protocol which uses NPI-based
    injection - sets up bit fields in the tag manually to account for
    a small difference (destination port offset) between Ocelot and
    Seville. Namely, ocelot_ifh_set_dest() is omitted out of
    ocelot_xmit_common(), because there's also seville_ifh_set_dest().

(2) ocelot_ifh_set_basic(), called by:
    - ocelot_fdma_prepare_skb() for FDMA transmission of the ocelot
      switchdev driver
    - ocelot_port_xmit() -> ocelot_port_inject_frame() for
      register-based transmission of the ocelot switchdev driver
    - felix_port_deferred_xmit() -> ocelot_port_inject_frame() for the
      DSA tagger ocelot-8021q when it must transmit PTP frames (also
      through register-based injection).
    sets the bit fields according to its own logic.

The problem is that (2) doesn't call ocelot_ifh_set_qos_class().
Copying that logic from ocelot_xmit_common() fixes that.

Unfortunately, although desirable, it is not easily possible to
de-duplicate code paths (1) and (2), and make net/dsa/tag_ocelot.c
directly call ocelot_ifh_set_basic()), because of the ocelot/seville
difference. This is the "minimal" fix with some logic duplicated (but
at least more consolidated).

Fixes: 0a6f17c6 ("net: dsa: tag_ocelot_8021q: add support for PTP timestamping")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>