gpio: 74xx: Fix build warning about void to integer cast

This fixes the build warning , warning: cast from pointer to integer
of different size when building this file on a x86 allmodconfig
configuration. In order for me to fix this build warning I changed
the cast in the function mmio_74xx_gpio_probe from casting the
variable data of the stucture pointer of_id to uintptr_t rather
then unsigned when assigning to the variable flag of the structure
pointer priv of the structure type mmio_74xx_gpio_priv.

Signed-off-by: Nicholas Krause <xerofoify@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>