sata_promise: fix endianess bug in ASIC PRD bug workaround

The original workaround for the Promise ASIC PRD bug
contained an endianess bug which I failed to detect:
the adjustment of the last PRD entry's length field
applied host arithmetic to little-endian data, which
is incorrect on big-endian machines.

We have the length available in host-endian format, so
do the adjustment on host-endian data and then convert
and store it in the PRD entry's little-endian data field.

Thanks to an anonymous reviewer for detecting this bug.
Signed-off-by: Mikael Pettersson <mikpe@it.uu.se>
Signed-off-by: Jeff Garzik <jeff@garzik.org>

drivers/ata/sata_promise.c

@@ -587,7 +587,7 @@ static void pdc_fill_sg(struct ata_queued_cmd *qc)
 			VPRINTK("Splitting last PRD.\n");
 
 			addr = le32_to_cpu(ap->prd[idx - 1].addr);
-			ap->prd[idx - 1].flags_len -= cpu_to_le32(SG_COUNT_ASIC_BUG);
+			ap->prd[idx - 1].flags_len = cpu_to_le32(len - SG_COUNT_ASIC_BUG);
 			VPRINTK("PRD[%u] = (0x%X, 0x%X)\n", idx - 1, addr, SG_COUNT_ASIC_BUG);
 
 			addr = addr + len - SG_COUNT_ASIC_BUG;