serial: sunzilog: Return proper error code from console ->setup() hook

For unifying console ->setup() handling, which is poorly documented,
return error code, rather than non-zero arbitrary number.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20200618164751.56828-4-andriy.shevchenko@linux.intel.com