i2c: mux: Remove class argument from i2c_mux_add_adapter()

99a741aa7a2d ("i2c: mux: gpio: remove support for class-based device
instantiation") removed the last call to i2c_mux_add_adapter() with a
non-null class argument. Therefore the class argument can be removed.

Note: Class-based device instantiation is a legacy mechanism which
shouldn't be used in new code, so we can rule out that this argument
may be needed again in the future.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Peter Rosin <peda@axentia.se>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>