net: dsa: mt7530: error out on failed reads in ATC/VTCR command polling

net: dsa: mt7530: error out on failed reads in ATC/VTCR command polling

mt7530_fdb_cmd() and mt7530_vlan_cmd() poll the command register
through a helper which returns 0 when the underlying read fails. A
failed bus transaction thus clears ATC_BUSY/VTCR_BUSY and is treated
as successful command completion, and the subsequent ATC_INVALID and
VTCR_INVALID checks are defeated the same way.

Poll using regmap_read_poll_timeout(), which stops on read errors and
propagates them, and check the completion status read as well. Take
the MDIO bus lock across the sequence as the switch regmap is set up
with locking disabled.

Fixes: b8f126a8d543 ("net-next: dsa: add dsa support for Mediatek MT7530 switch")
Fixes: 83163f7dca56 ("net: dsa: mediatek: add VLAN support for MT7530")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/eea1d8f15c54375b3770c23e09fb3217df487169.1785213071.git.daniel@makrotopia.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>