Input: synaptics - avoid using uninitialized variable when probing

commit f39f8688888ae74fa8deae2d01289b69b4727394 upstream.

synaptics_detect() does not check whether sending commands to the
device succeeds and instead relies on getting unique data from the
device. Let's make sure we seed entire buffer with zeroes to make sure
we will not use garbage on stack that just happen to be 0x47.

Reported-by:  <syzbot+13cb3b01d0784e4ffc3f@syzkaller.appspotmail.com>
Reviewed-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>