Input: cyttsp4_core - fix use after free bug

The device md->input is used after it is released. Setting the device
data to NULL is unnecessary as the device is never used again. Instead,
md->input should be assigned NULL to avoid accessing the freed memory
accidently. Besides, checking md->si against NULL is superfluous as it
points to a variable address, which cannot be NULL.

Signed-off-by: Pan Bian <bianpan2016@163.com>
Link: https://lore.kernel.org/r/1572936379-6423-1-git-send-email-bianpan2016@163.com


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>