pata_hpt366: add hpt36x_find_mode() helper

Factor out code for finding the register programming information
from hpt366_set_mode() to hpt36x_find_mode().

This makes pata_hpt366 driver more similar to pata_{37x,3x2n} ones.

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