drivers: wifi: siwx91x: Align the ways errors are managed

Unify the way the error code are tested:
  - sl_status_t is an alias to an int. The compiler won't complain if an
    int is assigned or compared to a sl_status_t. In the other hand,
    "int" is well known to manage error code. So, just use "int"
    everywhere.
  - Always name the return code "ret"
  - Unless it is meaningful, test the value of ret against 0
    (SL_STATUS_OK is just a convoluted way to say 0).

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>