ata: pata_hpt37x: merge hpt374_read_freq() to hpt37x_pci_clock()

With hpt374_read_freq() implemented as a separate function, there's
some code duplication going on, not to mention that this function is
named incorrectly: it returns the f_CNT register value saved by BIOS,
not the PCI clock frequency.
Folding hpt374_read_freq() into hpt37x_pci_clock() saves 20 bytes of
object code with x86_64 gcc 10.3.1...

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