HID: hiddev: use hid_hw_power instead of usbhid_get/put_power

Instead of calling into usbhid code directly, let's use the standard
accessors for the transport HID drivers, and stop clobbering their error
codes with -EIO.

This also allows us to remove usbhid_get/put_power(), leaving only
usbhid_power().

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>