gpio: brcmstb: Return proper error if bank width is invalid

Return proper error in brcmstb_gpio_probe if bank width is invalid.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Gregory Fong <gregory.0xf0@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>