atp: fix mouse attachment on PowerBook5,6 (G4)

Skip device mode switch step on Fountain-based devices as they don't
support RAW_SENSOR_MODE command, so failing to attach.

This was reproduced on PowerBook G4 (model PowerBook5,6) equipped with
product ID 0x020e

Reviewed by: hselasky
Approved by: jhibbits (mentor)
Differential Revision: https://reviews.freebsd.org/D24005