ata_timing: ensure t->cycle is always correct

Russell King hit a case where quantisation errors accumulated such that
the cycle time was shorter than rather than equal to the active/recovery
time. The code already knows how to stretch times to fit the cycle time
but does not know about the reverse.

Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>