libata: remove irq_on from ata_bus_reset() and ata_std_postreset()

  It seems irq_on() in ata_bus_reset() and ata_std_postreset()
are leftover of the EDD reset. Remove them.

Signed-off-by: Albert Lee <albertcc@tw.ibm.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>