The check for !dev_data->domain in __detach_device() emits a warning and
returns. The calling code in detach_device() dereferences dev_data->domain
afterwards unconditionally, so in case that dev_data->domain is NULL the
warning will be immediately followed by a NULL pointer dereference.

The calling code in cleanup_domain() loops infinite when !dev_data->domain
and the check in __detach_device() returns immediately because dev_list is
not changed.

do_detach() duplicates this check without throwing a warning.

Move the check with the explanation of the do_detach() code into the caller
detach_device() and return immediately. Throw an error, when hitting the
condition in cleanup_domain().
Signed-off-by: Anna-Maria Gleixner <anna-maria@linutronix.de>
Signed-off-by: Joerg Roedel <jroedel@suse.de>