[libata irq-pio] reorganize ata_pio_sector() and __atapi_pio_bytes()

   - move some code out of the kmap_atomic() / kunmap_atomic() zone
   - remove the redundant "do_write = (qc->tf.flags & ATA_TFLAG_WRITE);"

Signed-off-by: Albert Lee <albertcc@tw.ibm.com>