llext: hotfix: fix function pointer logging

Some function pointers were being passed via `%p` to LOG_DBG, and this
was causing the following issues in SOF CI with the `sparse` checker:

   subsys/llext/llext.c: error: arithmetics on pointers to functions
   subsys/llext/llext.c: error: incompatible types for operation (+)

This patch fixes the issue by casting the function pointers to void*.
Also fix a misleading error message in `llext_get_fn_table()`.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>