Document what the fix-up is does and make it more robust by ensuring
that it is only applied to the USB interface that corresponds to the
mouse (sony_report_fixup() is called once per interface during probing).

Cc: linux-input@vger.kernel.org
Cc: linux-usb@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: NFernando Luis Vazquez Cao <fernando@oss.ntt.co.jp>
Signed-off-by: NJiri Kosina <jkosina@suse.cz>