i2c: export i2c_client_type structure

i2c bus has 2 different types of device belonging to the same bus and
bus notifiers use device type to distinguish between adapters and clients.
Previously we only had i2c_adapter_type exported, which made code wanting
to work with i2c_client devices test for type not equal to adapter type.
This unfortunately is not safe if we ever add another type to the bus,
so let's export i2c_client_type as well.

Reviewed-by: Jean Delvare <jdelvare@suse.de>
Acked-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Reviewed-by: Wolfram Sang <wsa@the-dreams.de>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>