pata_hpt37x: actually limit HPT370 to UltraDMA/66

The driver clearly tries to limit HPT370[A] to UltraDMA/66 if the PCI clock is
less than 50 MHz but due to cut&paste type mistake this code has no effect...

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