gpio: pca953x: Deduplicate the bank_shift

The bank_shift = fls(...) code was duplicated in the driver 5 times,
pull it into separate function.

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