In an effort to kill libata-specific assert() and use generic
WARN_ON(), this patch converts all assert(X)'s in libata core layer to
WARN_ON(!X)'s.  Most conversions are straight-forward logical negation
exception for the followings.

* In libata-core.c:ata_fill_sg(),
  assert(qc->n_elem > 0) is converted to WARN_ON(qc->n_elem == 0) because
  qc->n_elem is unsigned and unsigned <= 0 is weird.

* In libata-scsi.c:ata_gen_ata_desc/fixed_sense(),
  assert(NULL != qc->ap->ops->tf_read) is converted to
  WARN_ON(qc->ap->ops->tf_read == NULL), as there are no other users of
  'constant cond var' style in libata.
Signed-off-by: NTejun Heo <htejun@gmail.com>
Signed-off-by: NJeff Garzik <jgarzik@pobox.com>