net: phy: broadcom: stub c45 read/write for 54810

The 54810 does not support c45. The mmd_phy_indirect accesses return
arbirtary values leading to odd behavior like saying it supports EEE
when it doesn't. We also see that reading/writing these non-existent
MMD registers leads to phy instability in some cases.

Fixes: b14995ac ("net: phy: broadcom: Add BCM54810 PHY entry")
Signed-off-by: Justin Chen <justin.chen@broadcom.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Link: https://lore.kernel.org/r/1691901708-28650-1-git-send-email-justin.chen@broadcom.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>