gpio-button-hotplug: use gpio_button_get_value() to initialize last_state.

TL-WR720N-v3 has a slider switch composed of 2 GPIO buttons which can be
used to swtich between 3 positions.  At leat 1 button is in pressed state
in any of those positions.  Initialize 'last_state' as 0 (released) will
cause the device to automatically enter failsafe mode on every bootup.

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@37643 3c298f89-4303-0410-b956-a3cf2f4a3e73