Input: gpio_keys_polled - keep button data constant

Commit 633a21d8 ("input: gpio_keys_polled: Add support for GPIO
descriptors") placed gpio descriptor into gpio_keys_button structure, which
is supposed to be part of platform data and not modifiable by the driver.
To keep the data constant, let's move the descriptor to
gpio_keys_button_data structure instead.

Tested-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>