gpio: bcm281xx: Fix return value of bcm_kona_gpio_get()

We need to return the corresponding bit for a particular GPIO. This bit
contains shift not mask.

Signed-off-by: Markus Mayer <markus.mayer@linaro.org>
Reviewed-by: Tim Kryger <tim.kryger@linaro.org>
Reviewed-by: Matt Porter <matt.porter@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>