[PATCH] libata: determine the err_mask when the error is found

  - move "qc->err_mask |= AC_ERR_ATA_BUS" to where the error is found
  - add "assert(qc->err_mask)" to ata_pio_error() to make sure qc->err_mask was available when we enter the error state

Signed-off-by: Albert Lee <albertcc@tw.ibm.com>
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Signed-off-by: Jeff Garzik <jgarzik@pobox.com>