i2c: mux: Avoid potential false error message in i2c_mux_add_adapter

I2C_CLASS_DEPRECATED is a flag and not an actual class.
There's nothing speaking against both, parent and child, having
I2C_CLASS_DEPRECATED set. Therefore exclude it from the check.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Acked-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Wolfram Sang <wsa@kernel.org>