net: dsa: mv88e6xxx: workaround RGMII transmit delay erratum for 6320 family

net: dsa: mv88e6xxx: workaround RGMII transmit delay erratum for 6320 family

Implement the workaround for erratum
  3.3 RGMII timing may be out of spec when transmit delay is enabled
for the 6320 family, which says:

  When transmit delay is enabled via Port register 1 bit 14 = 1, duty
  cycle may be out of spec. Under very rare conditions this may cause
  the attached device receive CRC errors.

Signed-off-by: Marek Behún <kabel@kernel.org>
Cc: <stable@vger.kernel.org> # 5.4.x
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20250317173250.28780-8-kabel@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>