net: dsa: mv88e6xxx: Add fix for erratum 5.2 of 88E6393X family

net: dsa: mv88e6xxx: Add fix for erratum 5.2 of 88E6393X family

BugLink: https://bugs.launchpad.net/bugs/1953731
commit 93fd8207bed80ce19aaf59932cbe1c03d418a37d upstream.

Add fix for erratum 5.2 of the 88E6393X (Amethyst) family: for 10gbase-r
mode, some undocumented registers need to be written some special
values.

Fixes: de776d0d316f ("net: dsa: mv88e6xxx: add support for mv88e6393x family")
Signed-off-by: Marek Behún <kabel@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>