Input: synaptics-rmi4 - fix possible double free

The RMI4 function structure has been released in rmi_register_function
if error occurs. However, it will be released again in the function
rmi_create_function, which may result in a double-free bug.

Signed-off-by: Pan Bian <bianpan2016@163.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>