gpio: mxs: Get rid of external API call

gpio: mxs: Get rid of external API call

The MXS driver was calling back into the GPIO API from
its irqchip. This is not very elegant, as we are a driver,
let's just shortcut back into the gpio_chip .get() function
instead.

This is a tricky case since the .get() callback is not in
this file, instead assigned by bgpio_init(). Calling the
function direcly in the gpio_chip is however the lesser
evil.

Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Janusz Uzycki <j.uzycki@elproma.com.pl>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>