libata-sff: remove harmful BUG_ON from ata_bmdma_qc_issue

Remove harmful BUG_ON() from ata_bmdma_qc_issue(),
as it casts too wide of a net and breaks sata_mv.
It also crashes the kernel while doing the BUG_ON().

There's already a WARN_ON_ONCE() further down to catch
the case of POLLING for a BMDMA operation.
Signed-off-by: Mark Lord <mlord@pobox.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>
Cc: stable@kernel.org

drivers/ata/libata-sff.c

@@ -2735,10 +2735,6 @@ unsigned int ata_bmdma_qc_issue(struct ata_queued_cmd *qc)
 {
 	struct ata_port *ap = qc->ap;
 
-	/* see ata_dma_blacklisted() */
-	BUG_ON((ap->flags & ATA_FLAG_PIO_POLLING) &&
-	       qc->tf.protocol == ATAPI_PROT_DMA);
-
 	/* defer PIO handling to sff_qc_issue */
 	if (!ata_is_dma(qc->tf.protocol))
 		return ata_sff_qc_issue(qc);