ata: pata_hpt37x: merge transfer mode setting methods

After commit e0afcf140e6e ("ata: pata_hpt37x: disable fast interrupts in
prereset() method") HPT370's and HPT372+'s PIO/DMA mode setting functions
have become identical -- merge them.

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