pinctrl: sirf: fix typo for GPIO bank number

The patch 7420d2d0: "pinctrl: sirf: switch driver to use gpiolib
irqchip helpers" from Apr 15, 2014, leads to the following static
checker warning:

      drivers/pinctrl/sirf/pinctrl-sirf.c:578 sirfsoc_gpio_handle_irq()
      warn: buffer overflow 'sgpio_chip.sgpio_bank' 5 <= 31

Cc: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Barry Song <Baohua.Song@csr.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>