pata_it8213: fix PIO2 underclocking

[ port of Sergei's fixes for pata_efar from commit 5f33b3bc ]

Fix the PIO mode 2 using mode 0 timings -- this driver should enable the
fast timing bank starting with PIO2, just like the PIIX/ICH drivers do.
Also, fix/rephrase some comments while at it.

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>