pinctrl: st: Fix irqmux handler

st_gpio_irqmux_handler() reads the status register to find out
which banks inside the controller have pending IRQs.
For each banks having pending IRQs, it calls the corresponding handler.

Problem is that current code restricts the number of possible banks inside the
controller to ST_GPIO_PINS_PER_BANK. This define represents the number of pins
inside a bank, so it shouldn't be used here.

On STiH407, PIO_FRONT0 controller has 10 banks, so IRQs pending in the two
last banks (PIO18 & PIO19) aren't handled.

This patch replace ST_GPIO_PINS_PER_BANK by the number of banks inside the
controller.

Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: <stable@vger.kernel.org> #v3.15+
Acked-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Maxime Coquelin <maxime.coquelin@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>