gpio: pca953x: Unify pca953x_{read,write}_regs_{8,mul}()

At this point, the pca953x_{read,write}_regs_mul() can read single bank
PCA953x GPIO chips as well. Merge the _8 and _mul functions together to
simplify the code a bit.

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>