gpio: max732x: Add missing dev reference to gpiochip

In case the gpiochip doesn't have the .dev field set, as is the case
in here, it is not possible to reference this device in DT as a GPIO
controller. A good example of this problem is that gpio-leds can not
be used when connected to this chip, the gpio-leds driver bails out
with -EPROBE_DEFER.

Fix this problem by setting the .dev field of the gpio_chip to the
parent i2c device.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Alexandre Courbot <gnurou@gmail.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Mans Rullgard <mans@mansr.com>
Cc: Olaf Mandel <o.mandel@menlosystems.com>
Cc: Semen Protsenko <semen.protsenko@globallogic.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>