gpio: max732x: use an inline function for container cast

Cast the struct gpio_chip into a max732x_chip using an inline
macro and move the assignment to the variable declaration
to save lines and simplify things.

Cc: Semen Protsenko <semen.protsenko@globallogic.com>
Acked-by: Mans Rullgard <mans@mansr.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>