net: mdio: mdio-i2c: Add support for single-byte SMBus operations

net: mdio: mdio-i2c: Add support for single-byte SMBus operations

PHYs that are within copper SFP modules have their MDIO bus accessible
through address 0x56 (usually) on the i2c bus. The MDIO-I2C bridge is
desgned for 16 bits accesses, but we can also perform 8bits accesses by
reading/writing the high and low bytes sequentially.

This commit adds support for this type of accesses, thus supporting
smbus controllers such as the one in the VSC8552.

This was only tested on Copper SFP modules that embed a Marvell 88e1111
PHY.

Tested-by: Sean Anderson <sean.anderson@linux.dev>
Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20250322075745.120831-3-maxime.chevallier@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>