i2c: mux: pca9541: remove support for unused platform data

There are no in-tree users of the platform data, so remove it to
simplify the code slightly.

Remove the now unused pca954x.h platform data header.

Signed-off-by: Robert Shearman <robert.shearman@att.com>
Signed-off-by: Peter Rosin <peda@axentia.se>