input: rename the internal callback struct to input_callback

Rename the internal input callback structure to input_callback. This is
for coherency with "INPUT_CALLBACK_DEFINE" and other similar code paths
in Zephyr, and also to avoid confusion with terminology.

This is an internal structure, applications should not have any
references to it so there should be no need for any release note
entries.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>