HID: alps: ALPS_1657 is too specific; use U1_UNICORN_LEGACY instead

HID_DEVICE_ID_ALPS_1657 PID is too specific, as there are many other
ALPS hardware IDs using this particular touchpad.

Rename the identifier to HID_DEVICE_ID_ALPS_U1_UNICORN_LEGACY in order
to describe reality better.

Fixes: 640e403b

 ("HID: alps: Add AUI1657 device ID")
Reported-by: Xiaojian Cao <xiaojian.cao@cn.alps.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>