i2c: mux: demux-pinctrl: use proper parent device for demux adapter

Due to a typo, the wrong parent device was assigned to the newly created
demuxing adapter device. It got connected to the demuxing platform
device but not to the selected parent I2C adapter device. Fix it to get
a proper parent-child relationship of the demuxed busses.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Peter Rosin <peda@axentia.se>