net: phy: marvell: Fix 88e1510 downshift counter errata

net: phy: marvell: Fix 88e1510 downshift counter errata

The 88e1510 PHY has an erratum where the phy downshift counter is not
cleared after phy being suspended(BMCR_PDOWN set) and then later
resumed(BMCR_PDOWN cleared). This can cause the gigabit link to
intermittently downshift to a lower speed.

Disabling and re-enabling the downshift feature clears the counter,
allowing the PHY to retry gigabit link negotiation up to the programmed
retry count times before downshifting. This behavior has been observed
on copper links.

Signed-off-by: Rohan G Thomas <rohan.g.thomas@altera.com>
Reviewed-by: Matthew Gerlach <matthew.gerlach@altera.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20250906-marvell_fix-v2-1-f6efb286937f@altera.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>