wlcore: Fix the return value in case of error in 'wlcore_vendor_cmd_smart_config_start()'

We return 0 unconditionally at the end of
'wlcore_vendor_cmd_smart_config_start()'.
However, 'ret' is set to some error codes in several error handling paths
and we already return some error codes at the beginning of the function.

Return 'ret' instead to propagate the error code.

Fixes: 80ff8063 ("wlcore: handle smart config vendor commands")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>