gpio: pl061: refactor type setting

Refactor this function so that I can understand it, do one
big read/modify/write operation and have the bitmask in a
variable instead of recalculating it every time it's needed.

Cc: Haojian Zhuang <haojian.zhuang@linaro.org>
Cc: Deepak Sikri <deepak.sikri@st.com>
Acked-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>