gpio: max732x: Drop unused support for irq and setup code via platform data

The only user of max732x_platform_data is arch/arm/mach-pxa/littleton.c
and it only uses .gpio_base. So drop the other members from the data struct
and simplify the driver accordingly.

The motivating side effect of this change is that the .remove() callback
cannot return a nonzero error code any more which prepares making i2c
remove callbacks return void.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>