HID: google: Enable PM Full On mode when adjusting backlight

hammer LED backlight brightness is not getting set when USB
device is in suspend state.

This patch fixes the issue by requesting USB HID device to be
in FULLON mode, so that sending hardware output report and
hardware raw request won't fail to set brightness, and set
device back to NORMAL mode once this call returns.

Signed-off-by: Haridhar Kalvala <haridhar.kalvala@intel.com>
Reviewed-by: Dmitry Torokhov <dtor@chromium.org>
Signed-off-by: Nicolas Boichat <drinkcat@chromium.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>