>>About the chunk you didn't understand.
>>The function must return an unsigned int, therefore "return -ENODEV;" is
>>wrong.
> 
> 
> ->init_chipset method returns either small positive number (irq) or BIG
> positive number in case of error ((unsigned int) -ENODEV). In
> do_ide_setup_pci_device() function ->init_chipset return value is casted to int
> and checked for "< 0".
> 

I made a HUGE mistake then. Sorry.
Thank you Alexey!!