remoteproc: st: Fix sometimes uninitialized ret in st_rproc_probe()

Clang warns (or errors with CONFIG_WERROR=y):

  drivers/remoteproc/st_remoteproc.c:357:6: error: variable 'ret' is used uninitialized whenever 'if' condition is true [-Werror,-Wsometimes-uninitialized]
    357 |         if (!ddata->config)
        |             ^~~~~~~~~~~~~~
  drivers/remoteproc/st_remoteproc.c:442:9: note: uninitialized use occurs here
    442 |         return ret;
        |                ^~~
  drivers/remoteproc/st_remoteproc.c:357:2: note: remove the 'if' if its condition is always false
    357 |         if (!ddata->config)
        |         ^~~~~~~~~~~~~~~~~~~
    358 |                 goto free_rproc;
        |                 ~~~~~~~~~~~~~~~
  drivers/remoteproc/st_remoteproc.c:348:9: note: initialize the variable 'ret' to silence this warning
    348 |         int ret, i;
        |                ^
        |                 = 0
  1 error generated.

Set ret to -ENODEV, which seems to be a standard return code w...