HID: hid-input: fix stylus battery reporting

With commit 4f388217 hid-input started clearing of "ignored" usages
to avoid using garbage that might have been left in them. However
"battery strength" usages should not be ignored, as we do want to
use them.

Fixes: 4f388217 ("HID: hid-input: clear unmapped usages")
Reported-by: Kenneth Albanowski <kenalba@google.com>
Tested-by: Kenneth Albanowski <kenalba@google.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>