gpio: pca953x: Zap single use of pca953x_read_single()

Drop pca953x_write_single() which is used in one place.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>