pata_hpt3x2n: calculate average f_CNT

Allow hpt3x2n_pci_clock() to calculate the average f_CNT register value iff
HighPoint BIOS hasn't saved one, just like the 'pata_hpt37x' driver (reading
the full 16-bit register, unlike what that driver does), so that this driver
would work correctly on e.g. non-x86 machine with 66 MHz PCI.

I'm not sure why Alan has only done this in one driver and not the other...

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