cmd64x: add/fix enablebits (take 2)

The IDE core looks at the wrong bit when checking if the secondary channel is
enabled on PCI0646 -- CNTRL register bit 7 is read-ahead disable, bit 3 is the
correct one.
Starting with PCI0646U chip, the primary channel can also be enabled/disabled --
so, add 'enablebits' initializers to each 'ide_pci_device_t' structure, handling
the original PCI0646 via adding the init_setup() method and clearing the 'reg'
field there if necessary...

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