pata_efar: fix register naming used in efar_set_piomode()

Rename 'idetm_port' and 'idetm_data' variables to 'master_port'
and 'master_data' respectively to match register naming used in
efar_set_dmamode() and in ata_piix.c.

Fix efar_set_piomode() documentation while at it.

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>