ata: pata_hpt37x: factor out hpt37x_pci_clock()

Factor out the PCI clock frequency detection code into hpt37x_pci_clock(),
so that this driver becomes more like 'pata_hpt3x2n'.  Note that I decided
to change the way HPT374 is identified to using the PCI device ID...

Signed-off-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>