Merge branch 'net-dsa-mt7530-fix-swallowed-mdio-read-errors'

Daniel Golle says:

====================
net: dsa: mt7530: fix swallowed MDIO read errors

While working on a register access cleanup for the mt7530 driver, the
Sashiko AI reviewers flagged long-standing error handling gaps in the
driver's read paths [1].

The MDIO regmap backend truncates negative bus->read() errnos into u16
register halves and returns success, handing garbage data to callers
and to read-modify-write cycles which then write it back to the switch.

The ATC/VTCR command polls and the MT7531 indirect PHY polls consume
reads through a helper which returns 0 when the underlying read fails.
A failed bus transaction thus clears the polled busy bit and is
mistaken for command completion, defeats the subsequent
ATC_INVALID/VTCR_INVALID checks the same way, and lets the indirect
PHY access functions return garbage PHY register data.

Fix the backend to propagate bus->read() errors, and convert the
command and PHY access polls to regmap_read_poll_timeout(), which
terminates polling on read errors and propagates them.

The cleanup series depending on these fixes will be submitted to
net-next separately after the next net/net-next merger.

[1] https://lore.kernel.org/netdev/cover.1784481922.git.daniel@makrotopia.org/
====================

Link: https://patch.msgid.link/cover.1785213071.git.daniel@makrotopia.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>