hpsa: fix 6-byte READ/WRITE with 0 length data xfer

a 6-byte READ/WRITE CDB with a 0 block data transfer really
means a 256 block data transfer.  The RAID mapping code failed
to handle this case.  For 10/12/16 byte READ/WRITEs, 0 just means
no data should be transferred, and should not trigger BUG_ON.

Signed-off-by: Stephen M. Cameron <scameron@beardog.cce.hp.com>
Reported-by: Robert Elliott <elliott@hp.com>
Reviewed-by: Robert Elliott <elliott@hp.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>