sata_promise: ASIC PRD table bug workaround, take 2

Second-generation Promise SATA controllers have an ASIC bug
which can trigger if the last PRD entry is larger than 164 bytes,
resulting in intermittent errors and possible data corruption.

Work around this by replacing calls to ata_qc_prep() with a
private version that fills the PRD, checks the size of the
last entry, and if necessary splits it to avoid the bug.
Also reduce sg_tablesize by 1 to accommodate the new entry.

Tested on the second-generation SATA300 TX4 and SATA300 TX2plus,
and the first-generation PDC20378.

Thanks to Alexander Sabourenkov for verifying the bug by
studying the vendor driver, and for writing the initial patch
upon which this one is based.

Signed-off-by: Mikael Pettersson <mikpe@it.uu.se>
--
Changes since previous version:
* use new PDC_MAX_PRD constant to initialise sg_tablesize

 drivers/ata/sata_promise.c |   87 ++++++++++++++++++++++++++++++++++++++++++---
 1 files changed, 83 insertions(+), 4 deletions(-)
Signed-off-by: Jeff Garzik <jeff@garzik.org>