pata_at91: SMC settings calculation bugfixes, support for t6z and IORDY

* New code correctly calculates SMC registers values, adjusts calculated
  to admissible ranges, enlarges cycles when required and converts values
  into SMC's format.
* Support for TDF cycles (ATA t6z) and IORDY line added.
* Eliminate need in the initial_timing structure.
* Code cleanup.

Signed-off-by: Igor Plyatov <plyatov@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>