libata: change ATA_QCFLAG_DMAMAP semantics

ATA_QCFLAG_DMAMAP was a bit peculiar in that it got set during qc
initialization and cleared if DMA mapping wasn't necessary.  Make it
more straight forward by making the following changes.

* Don't set it during initialization.  Set it after DMA is actually
  mapped.

* Add BUG_ON() to guarantee that there is data to transfer if DMAMAP
  is set.  This always holds for the current code.  The BUG_ON() is
  for docummentation and sanity check.

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