[PATCH] libata: update ata_do_simple_cmd()

* the function has always returned AC_ERR_* masks not -errno but its
  return type was int.  Make return type unsigned int.

* don't print error message automatically.  it's the caller's
  responsibility.

* add header comment

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>