HID: multitouch: prevent memleak with the allocated name

mt_free_input_name() was never called during .remove():
hid_hw_stop() removes the hid_input items in hdev->inputs, and so the
list is therefore empty after the call. In the end, we never free the
special names that has been allocated during .probe().

Restore the original name before freeing it to avoid acessing already
freed pointer.

This fixes a regression introduced by 49a5a827 ("HID: multitouch: append " Pen" to
the name of the stylus input")

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>