i2c: stub: Don't let i2c adapters declare I2C_CLASS_SPD support if they support I2C_CLASS_HWMON

After removal of the legacy eeprom driver the only remaining I2C
client device driver supporting I2C_CLASS_SPD is jc42. Because this
driver also supports I2C_CLASS_HWMON, adapters don't have to
declare support for I2C_CLASS_SPD if they support I2C_CLASS_HWMON.
It's one step towards getting rid of I2C_CLASS_SPD mid-term.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>