gpio: mcp23s08: fix up compilation error

The driver depends on the chip.of_node being present to compile,
which is the case on some target platforms but not others.
Instead, rely on chip.dev->of_node to be used, as struct device
always has an of_node in place.

Cc: Alexander Stein <alexander.stein@systec-electronic.com>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>