piix: fix 82371MX enablebits

piix: fix 82371MX enablebits

According to the datasheet, Intel 82371MX (MPIIX) actually has only a
single IDE channel mapped to the primary or secondary ports depending on
the value of the bit 14 of the IDETIM register at PCI config.  offset 0x6C
(the register at 0x6F which the driver refers to.  doesn't exist).  So,
disguise the controller as dual channel and set enablebits masks/values
such that only either primary or secondary channel is detected enabled.
Also, preclude the IDE probing code from reading PCI BARs, this controller
just doesn't have them (it's not the separate PCI function like the other
PCI controllers), it only decodes the legacy addresses.

[ Alan sayeth " MPIIX does not work with or without the change.  It needs its
  own different driver and not to use setup-pci.  Huge job and since it works
  well with libata who cares.  Ditto the early PIIX chip." ]

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>