Input: st-keyscan - fix potential zalloc NULL dereference

This patch fixes the following static checker warning:

drivers/input/keyboard/st-keyscan.c:156 keyscan_probe()
error: potential zalloc NULL dereference: 'keypad_data->input_dev'

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Gabriel Fernandez <gabriel.fernandez@st.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>