[PATCH] ahci: simplify ahci_start_engine()

Simplify ahci_start_engine() by killing prerequisite condition checks.
Rationales are..

* No user checks error return from ahci_start_engine()

* Code flow guarantees the prerequisite conditions unless the
  controller is malfunctioning.  In such cases, the driver had chances
  to learn about the problem _before_ calling this function.

* Closely related to the above two, driver calls into this function
  even when prerequisites fail hoping for the best.

Basically, ahci_start_engine() should only do the operation itself.
It isn't the right place to check for prerequisites.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Zhao, Forrest <forrest.zhao@intel.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>