i2c: mux: pca954x: use helper variable in probe

This creates a struct device *dev helper variable in probe()
which IMO makes the code less cluttered and easier to read.
Also rename the of_node veriable to the common shortform "np"
(node pointer).

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
[peda: slightly edited commit message and removed some surplus newlines]
Signed-off-by: Peter Rosin <peda@axentia.se>