USB: sisusbvga: fix oops in error path of sisusb_probe

The pointer used to log a failure of usb_register_dev() must
be set before the error is logged.

v2: fix that minor is not available before registration

Signed-off-by: oliver Neukum <oneukum@suse.com>
Reported-by: syzbot+a0cbdbd6d169020c8959@syzkaller.appspotmail.com
Fixes: 7b5cd5fefbe02 ("USB: SisUSB2VGA: Convert printk to dev_* macros")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
