gpio: brcmstb: correct the configuration of level interrupts

This commit corrects a bug when configuring the GPIO hardware for
IRQ_TYPE_LEVEL_LOW and IRQ_TYPE_LEVEL_HIGH interrupt types. The
hardware is now correctly configured to support those types.

Fixes: 19a7b694 ("gpio: brcmstb: Add interrupt and wakeup source support")
Signed-off-by: Doug Berger <opendmb@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Gregory Fong <gregory.0xf0@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>