HID: intel-ish-hid: Make remove callback return void

The driver core ignores the return value of struct bus_type::remove()
because there is only little that can be done. To simplify the quest to
make this function return void, let struct ishtp_cl_driver::remove() return
void, too. All users already unconditionally return 0, this commit makes
it obvious that returning an error value is a bad idea.

Signed-off-by: Uwe Kleine-König <uwe@kleine-koenig.org>
Acked-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>