cmd64x: fix recovery time calculation (take 3)

The driver wrongly takes the address setup time into account when calculating
the PIO recovery time -- this leads to slight overclocking of the PIO modes 0
and 1 (so, the prayers failed to help, as usual :-).  Rework the code to be
calculating recovery clock count as a difference between the total cycle count
and the active count (we don't need to calculate the recovery time itself since
it's not specified for the PIO modes 0 to 2, and for modes 3 and 4 this formula
gives enough recovery time anyway in the chip's supported PCI frequency range).

This patch has been inspired by reading the datasheets and looking at what the
libata driver does; it has been compile-tested only (as usual :-) but anyway,
the new code gives the same or longer recovery times than the old one...

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