ide-iops: only clear DMA words on setting DMA mode

The bytes indicating current DMA mode in the identify data words 62, 63, and 88
should only change on setting a DMA mode, so stop clearing them  on setting PIO
mode in ide_config_drive_speed().  While at it, correct SW/MW DMA mode masks...

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>