i2c: pca954x: put the mux to disconnected state after resume

pca954x may be power lost during suspend, so after resume we also suffer
the issue fixed by commit cd823db8,

 "pca954x power-on default is channel 0 connected. If multiple pca954x
 muxes are connected to the same physical I2C bus, the parent bus will
 see channel 0 devices behind both muxes by default."

What's more, when resume bootloader may also operate the mux, so the
the channel connected after that may not be the one driver thought.

We fix this problem by putting the mux to disconnected state and
clearing last_chan in the resume hook.

Signed-off-by: Jisheng Zhang <jszhang@marvell.com>
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>