pdc202xx_new: switch to using pci_get_slot() (take 2)

Switch to using pci_get_slot() in init_setup_pdc20270() to get to the mate chip
behind DC21150 bridge as there's no need for the driver itself to walk the list
of the PCI devices (and the driver didn't check the bus # of the found device).
While at it, make it emit warning about IRQ # being fixed up (just like hpt366.c
does) and "beautify" this whole function as well as init_setup_pdc20276()...

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>