[PATCH] libata: dec scmd->retries for qcs with zero err_mask

qcs might get retried because of unrelated failure.  e.g. NCQ command
failure causes the whole command set to be aborted.  Decrement
scmd->retries for such retrials to avoid unnecessarily failing
commands.  Note that scmd->retries will be incremented the first time.

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