net: dsa: mt7530: check bus->read() errors in the MDIO regmap backend

net: dsa: mt7530: check bus->read() errors in the MDIO regmap backend

bus->read() returns a negative errno on failure, but
mt7530_regmap_read() assigns it to a u16, truncating e.g. -ETIMEDOUT
into 0xff92, and returns success. The garbage word is then consumed as
register data, and read-modify-write cycles write it back to the
switch. Check both reads and propagate their errors.

The same defect existed in mt7530_mii_read() since the driver was
introduced and moved into the regmap backend unchanged.

Fixes: b8f126a8d543 ("net-next: dsa: add dsa support for Mediatek MT7530 switch")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/3c628e48276c2e5522c8795a6be60d11c7a76a7d.1785213071.git.daniel@makrotopia.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>