gpio: dwapb: Call directly into the gpiochip to read value

We were going out through the (legacy) gpio API to read the value
of a line to set up polarity inversion. This is abusive. Do something
less abusive by looking up the actual struct gpio_chip *
instance and calling .get() directly on it.

Acked-by: Hoan Tran <hotran@apm.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>