i2c: muxes: pca954x: look up width from chip_desc

Commit 8e6eda7c ("drivers/i2c/muxes/pca954x: Add pca9547 I2C mux
support") introduced a chip_desc for the pca954x devices but failed to
update pca954x_ofdata_to_platdata() to be aware of it. Make
pca954x_ofdata_to_platdata() lookup the chip_desc to validate the device
width.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Marek Behun <marek.behun@nic.cz>