ide: filter out invalid transfer mode values in set_xfer_rate()

Filter out invalid transfer mode values (0x02-0x07) in write handler
for obsoleted /proc/ide/hd?/settings:current_speed setting.

While at it:

* Use XFER_UDMA_6 define.

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