serial: core: Fix error handling for serial_core_ctrl_device_add()

Checking for NULL is incorrect as serial_base_ctrl_add() uses ERR_PTR().

Let's also pass any returned error along, there's no reason to translate
all errors to -ENODEV.

Fixes: 84a9582f ("serial: core: Start managing serial controllers to enable runtime PM")
Signed-off-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20230602070007.59268-1-tony@atomide.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>