ht6560b can only do up to PIO mode 4

According to the datasheet, ht6560b only supports up to PIO mode 4.

[bart: manually ported it over 2.6.25-rc2]

Signed-off-by: Jan Evert van Grootheest <janevert@caiway.nl>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>