Input: rmi4 - refactor function allocation and registration

Input: rmi4 - refactor function allocation and registration

Currently, rmi_create_function() allocates memory for the rmi_function
structure, but rmi_register_function() initializes the device via
device_initialize(). This split of ownership makes error handling in
rmi_create_function() confusing because the caller must be aware that
if rmi_register_function() fails, it has already called put_device() to
clean up the memory.

To make the memory lifecycle explicit and fix potential leaks cleanly
introduce rmi_alloc_function() to handle memory allocation and device
initialization, and make the caller of rmi_register_function()
responsible for cleanup.

Assisted-by: Gemini:gemini-3.1-pro
Link: https://patch.msgid.link/20260505045952.1570713-10-dmitry.torokhov@gmail.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>