pata_hpt{37x|3x2n}: improve timing register documentation

Describe UDMA timing bits 18-20 and 21 separately; add a note to bit
31 about it being meaningful for PIO only.  Reformat the whole comment,
while at it...

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>