cmd64x: init. code cleanup

Fix two minor issues with PCI0646 chip reporting in the init_chipset() method:
"IRQ workaround enabled" message printed out not only for revision 0x01 and
"CMD646: chipset revision" printed twice (by IDE core and the driver itself).
Also, remove empty/pointless switch cases for the chips other than PCI0646,
duplicate write to the MRDMODE register when enabling interrupts and MEMORY
READ LINE cycles, and needless/misplaced initialization of the timing registers
in this method.
Switch to reading only the PCI revision ID register itself, not the whole 32
bits at its address in init_chipset() and init_hwif() methods; in addition,
get rid of the useless clearing of hwif->autodma and perform some cosmetic
style changes in the latter method.
Refactor ata66_cmd64x() by moving all the code into the 'switch' statement,
renaming/adding variables, and fixing the coding style.
While at it, finally get rid of the meaningless aliasing register #define's...

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