HID: multitouch: fix name of Stylus input devices

HID_DG_PEN devices should have a suffix of "Stylus", as pointed out by
commit c0ee1d571626 ("HID: hid-input: Add suffix also for HID_DG_PEN").
However, on multitouch devices, these suffixes may be overridden. Before
that commit, HID_DG_PEN devices would get the "Stylus" suffix, but after
that, multitouch would override them to have an "UNKNOWN" suffix. Just add
HID_DG_PEN to the list of non-overriden suffixes in multitouch.

Before this fix:

[ 0.470981] input: ELAN9008:00 04F3:2E14 UNKNOWN as /devices/pci0000:00/0000:00:15.1/i2c_designware.1/i2c-16/i2c-ELAN9008:00/0018:04F3:2E14.0001/input/input8
ELAN9008:00 04F3:2E14 UNKNOWN

After this fix:

[ 0.474332] input: ELAN9008:00 04F3:2E14 Stylus as /devices/pci0000:00/0000:00:15.1/i2c_designware.1/i2c-16/i2c-ELAN9008:00/0018:04F3:2E14.0001/input/input8

ELAN9008:00 04F3:2E14 Stylus

Fixes: c0ee1d571626 ("HID: hid-input: Add suffix also for HID_DG_PEN")
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@igalia.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>