isci: move task context alignment from run-time to compile time

Remove usage of PTR_ALIGN by arranging for the task context to be aligned by
the compiler.  Another step towards unifying isci_request and
scic_sds_request.  Once this is complete the task context in the request can
likely be removed in favor of building the task directly to tc memory (see:
scic_sds_controller_copy_task_context).  It's not clear why this needs to be
cacheline aligned if we just end up copying before submission...

Signed-off-by: Dan Williams <dan.j.williams@intel.com>