gpio-button-hotplug: leave platform_device.dev.platform_data untouched

gpio-button-hotplug: leave platform_device.dev.platform_data untouched

get platform_data from gpio_keys_button_dev.pdata, and fix a illegal pointer
dereference like this:

[   51.143776] gpio-keys-polled gpio-keys-polled: missing poll_interval value
[   51.150852] gpio-keys-polled: probe of gpio-keys-polled failed with error -22
[  828.159993] gpio-keys-polled gpio-keys-polled: no memory for button data
[  828.166821] gpio-keys-polled: probe of gpio-keys-polled failed with error -12

Signed-off-by: Furong Xu <xfr@outlook.com>